ITF 3-5 text.txt

Non-volatile computer storage is based around mass storage drives. Every computer comes with a primary fixed disk (HDD or SSD). This stores the operating system and applications software that has been installed to the PC plus data files created by users. The computer may also have a number of other...

Non-volatile computer storage is based around mass storage drives. Every computer comes with a primary fixed disk (HDD or SSD). This stores the operating system and applications software that has been installed to the PC plus data files created by users. The computer may also have a number of other storage devices, such as a secondary HDD or SSD, a CD/DVD/BD optical drive or writer, USB removable drives, or a flash memory card reader. In order for the OS to able to read and write files to a drive, it must be partitioned and formatted with a file system. Hard Disk Partitions Partitioning a hard disk is the act of dividing it into logically separate storage areas. This may be done to improve the performance of the disk, to install multiple operating systems, or to provide a logical separation of different data areas. You must create at least one partition on the hard disk before performing a format to create a file system. Typically, this is done through Windows Setup when building a new PC or through Disk Management when adding an extra hard disk. Historically, removable media such as USB thumb drives can only be formatted with a single partition using the Disk Management tool provided with Windows. However, the Windows 10 1703 update added support for creating multiple partitions on removable drives. External hard drives usually come preconfigured with a partition and file system. On the primary fixed disk, one of the partitions must be made active. This active partition is also referred to as the system partition. An active partition is used by the computer to boot. In Windows, the system partition is usually hidden from File Explorer and is not allocated a drive letter. Windows Drives In Windows, each formatted partition can be allocated a drive letter, from A through Z. The boot partition (containing the operating system files) is usually allocated the letter C. Each removable drive (CD/DVD/BD or flash memory card for instance) can also be allocated a drive letter. hard disk; 2) D: is an optical drive; 3) E: represents an external USB-attached hard drive; 4) I: has been assigned to an SD memory card. Screenshot used with permission from Microsoft. File Systems Each partition can be formatted with a different file system. Under Windows, there is a choice between FAT and NTFS. FAT (File Allocation Table)---this was used for older versions of Windows and is preserved under Windows for compatibility. Typically, the 32-bit version (FAT32) is used. This permits a maximum file size of 4 GB and a maximum partition size of 32 GB. NTFS (New Technology File System)---as a 64-bit addressing scheme, NTFS allows much larger partitions (up to 2 TB) than FAT. NTFS also supports extended attributes, allowing for file-level security permissions, compression, and encryption. These features make NTFS much more stable and secure than FAT. Windows must be installed to an NTFS partition. FAT32 is used for formatting most removable drives and disks as it provides the best compatibility between different types of computers and devices. CDs and DVDs are often formatted using Universal Disk Format (UDF), though the older CD format ISO 9660 (or CDFS) offers the best compatibility with legacy drives. Recordable media can be written to once only; rewritable media support deleting and adding files later, but to make the disc fully compatible with consumer DVD players, the session must be closed. Most Linux distributions use some version of the ext file system to format partitions on mass storage devices. ext3 is a 64-bit file system with support for journaling, which means that the file system tracks changes, giving better reliability and less chance of file corruption in the event of crashes or power outages. Support for journaling is the main difference between ext3 and its predecessor (ext2). ext4 delivers significantly better performance than ext3 and would usually represent the best choice for new systems. Apple Mac workstations and laptops use the extended Hierarchical File System (HFS+), though the latest macOS version is being updated to the Apple File System (APFS). File System Features You can evaluate file systems by considering which features they do or do not support: Compression---the file system can automatically reduce the amount of disk space taken up by a file. The file system applies a non-lossy algorithm to the file to find ways to store the data in it more efficiently without discarding any information. Note that file system compression only benefits files that are not already compressed. A file type such as JPEG that already applies compression will be significantly reduced in size. Encryption---the file system can automatically encrypt data in a file when it is saved. This means that the file can only be opened when there is access to the encryption key. If this is stored separately to the data and/or its use is protected by a password, the data on the drive is protected even if the disk is stolen and installed in another computer system. Permissions---the file system maintains an Access Control List (ACL) for each file or folder object. The ACL records which user accounts are allowed to read, write, or control the object. Journaling---the file system tracks changes or intended changes in a log. This means that if there is a sudden power cut and a particular write operation was interrupted, the journal may be used to recover the data or at least restore the file system to good working order (consistent state). Limitations---as noted in the table below, file systems have limits in terms of their maximum capacity and the size of individual files. Naming rules---very old file systems limited the size of a file name to eight characters plus a three-character extension. Modern file systems support longer file names (usually up to 255 characters) and complete directory paths, use of Unicode characters in the name, and support distinguishing the case of file name characters. File systems also have a number of reserved characters which cannot be used in a file name. Folders and Directories The purpose of a drive is to store files. Folders are a means of organizing files on each drive to make them easier to find. Folders can also create distinct areas in terms of security access controls. Operating system files can be separated from user data files, and standard users can be prevented from modifying them. Also, each user can have a protected storage area that other standard users cannot access, unless the folder is shared. Folders are created in a hierarchy of subfolders. The first level of the hierarchy is called the root folder. This is created when the drive is formatted. The root folder is identified by the drive label and a backslash. For example, the root folder of the C: drive is C: The root folder can contain files and subfolders. The path to a subfolder is also separated by backslashes. For example, in C:32, WINDOWS is a subfolder of the root and System32 is a subfolder of WINDOWS. Windows System Folders A default folder structure is created on the boot partition when Windows is installed. A default installation creates the following three system folders: Windows---the "system root," containing drivers, logs, add-in applications, system and Registry files (notably the System32 subfolder), and so on. System32 contains most of the applications and utilities used to manage and configure Windows. Program Files---subfolders for installed applications software. A 64-bit version of Windows uses a "Program Files" folder for 64-bit applications software and a "Program Files (x86)" folder for 32-bit applications software. Users---storage for users' profile settings and data (Documents, Temporary Internet Files, Cookies, recent file shortcuts, desktop shortcuts, and so on). The contents of Program Files and Windows should not be moved or changed except by using the proper Windows utilities and application installers. Moving or deleting files manually could cause serious problems. Ordinary users are denied access to system folders. Linux Directories "Folder" is a Windows-specific term. In Linux, these containers are called directories. Also, Linux uses the forward slash (/) to represent the root and as a directory delimiter. For example, in the directory path /home/andy, home is a subdirectory of the root directory and andy is a subdirectory of home. Note that a forward slash will also work in Windows if you type it into the address bar or use one in a file path at the command prompt. It is important to realize that everything available to the Linux OS is represented as a file in the file system, including devices. This is referred to as the unified file system. For example, a single hard drive attached to a SATA port would normally be represented in the file system by /dev/sda. A second storage device---perhaps one attached to a USB port---would be represented as /dev/sdb. There is no concept of "drive letters" in Linux. Everything is represented through the file system. Think of the root file system representing everything on the computer as "THE" file system and a file system for a particular partition as just "A" file system. A file system configured on a partition on a particular storage device is attached to a particular directory (mount point) within the unified file system using the mount command. For example: mount /dev/sda1 /mnt/mydrive...mounts partition 1 on the mass storage device sda to the directory /mnt/mydrive. Mountable file systems are listed in the /etc/fstab file. In Windows, File Explorer (called Windows Explorer in previous versions and very widely just referred to as "Explorer") provides a visual means of navigating the file system. In the main pane, you can double-click a folder to open it. You can use the Navigation pane to expand and collapse objects or the Breadcrumb on the address bar and Back and Forward buttons on the toolbar. Navigation Pane When browsing the computer using File Explorer in Windows 10, two top-level categories are shown in the navigation pane. Quick access contains shortcuts to folders that are most useful. These can be modified by dragging and dropping. By default, it contains shortcuts to your personal Desktop, Downloads, Documents, and Pictures folders. File Explorer navigation pane showing top-level categories. Screenshot used with permission from Microsoft. The second top-level category is the Desktop. Under the "Desktop" object, you can find the following categories: OneDrive---if you sign into the computer with a Microsoft account, this shows the files and folders saved to your cloud storage service on the Internet. As you can see from the screenshot, other cloud service providers may add links here too. User account---the folders belonging to your account profile. For example, in the screenshot above the user account is listed as "James at CompTIA." This PC---access to user-generated files in the user's profile plus the hard drives and removable storage drives available to the PC. Libraries---these can be used to create views of folders and files stored in different locations and on different disks. Network---contains computers, shared folders, and shared printers available over the network. Control Panel---options for configuring legacy Windows features (most configuration is now performed via the Settings app rather than Control Panel). Recycle Bin---provides an option for recovering files and folders that have been recently deleted. Earlier versions of Windows use different organizational principles for the top-level system objects. For example, Windows 7 has the user's name as the system object for accessing profile folders and the "Computer" object equivalent of "This PC" only shows drives. User Profiles and Libraries Each user has his or her own profile folder, stored under the Users system folder. Files in each user's profile are private (though a user with administrative privileges can still access them). Each profile folder contains subfolders for different types of file (documents, music, pictures, video, and so on). The profile folder also contains hidden subfolders used to store application settings and customizations, favorite links, shortcuts, temporary files, and so on. Windows also configures a Public profile to allow users of the PC to share files between them (a local share). Folders can also be made available over a network (a network share). See Unit 4.4 for more information about folder sharing. In Windows 10, libraries are used to provide easy access to different kinds of documents that may be stored in different places. For example, you may store pictures in your pictures folder, on a flash drive, and on a network. You can view all these pictures in one location by adding the locations to a library. Libraries work as a kind of "virtual" folder. By default, each profile contains libraries for Documents, Music, Pictures, and Videos. You can create new libraries using the toolbar or by right-clicking in the Libraries folder. Right-clicking a library icon allows you to set the locations (folders) it includes and optimize the library display settings for a particular type of file. You can also set the default save location (the physical folder used when you save a file to a library) The shortcut menus for folders have the Include in Library option, providing another way of adding them to a library. Creating a Folder You can use the shortcut or File menus to create a new folder within another object. Windows has various folder naming rules that must be followed when modifying the folder structure: No two subfolders within the same folder may have the same name. Subfolders of different folders may have the same name though. Folder names may not contain the following reserved characters:  / : \* ? \" \< \> \| The full path to an object (including any file name and extension) may not usually exceed 260 characters. A warning message is displayed if these rules are not followed and the user is prompted to enter a new folder name. Folder and file names are case aware, which means that the system preserves case in the name as entered but does not regard the case as significant for operations such as detecting duplicate names or indexing. Files Files are the containers for the data that is used and modified through the operating system and applications. Files store either text or binary data; text data is human-readable, while binary data can only be interpreted by a software application compatible with that file type. File Types and Extensions Files follow a similar naming convention to folders, except that the last part of the file name represents an extension, which describes what type of file it is and is used by Windows to associate the file with an application. The extension is divided from the rest of the file name by a period. By convention, extensions are three characters. By default, the extension is not shown to the user. Linux (and Android) use file permissions rather than extensions to determine whether a file is executable. Many Linux GUI file managers do use extensions to make opening a file within a suitable application easier though. You can use a period as part of the main part of the file name too. It is the last period that delimits the file extension. Creating and Opening Files System and application files are created when you install programs. User files are created when you use the Save or Save As function of a program. As you can see, the File Explorer tools are available in an application's Save dialog to navigate between folders. Most applications let you save the file in one of several file formats, accessed through the Save as type box. You can also create certain types of file in Explorer by right-clicking in a folder and selecting New, followed by the type of file you want to create. Files are usually opened by double-clicking them. You may want to open a file in a software product other than the default however. When you right-click a file, the shortcut menu displays a list of suitable choices, or you can choose Open With and browse for different application. You can also use the Default Programs applet to configure file associations. When creating and editing text files, you must be careful to use a plain text file format, such as that used by Notepad (a Windows accessory). If you convert a plain text system file to a binary format, it will become unusable. Some files can have different associations for the Open and Edit commands. For example, a picture file might open in the Photos app when you double-click it, but if you right-click and select Edit, it will open in Paint. You must also use the Save command to retain any changes you make while editing a document. If you want to keep both the original document and the edited version, use the Save As command to create a new file with a different name and/or stored in a different folder. File Explorer Options The File Explorer Options applet in Control Panel controls how Explorer works. The General tab contains options for opening files by single-clicking and for opening folders in the same or new windows. The View tab contains a long list of options affecting how folders and files are displayed in Explorer (such as whether to show hidden files or file extensions). View settings (such as whether to show thumbnail icons or details) are retained on a per-folder basis but can be reset using the buttons on the View tab. Renaming Files and Folders To rename a file or folder, select it, press F2, then type the new name. You can also right-click the file and select Rename. Do not change a file's extension if it is shown. If you delete or change the extension, the file will not be associated with the correct program for opening it. Copying and Moving Files Explorer supports multiple methods of moving or copying files. These include: Use the Edit \> Cut/Copy/Paste commands from the main menu or shortcut menu or their keyboard shortcuts (CTRL+X, CTRL+C, CTRL+V). Drag and drop the object, holding down CTRL to copy or SHIFT to move (or CTRL+SHIFT to create a shortcut). The default action (no key press) for drag and drop is to move the selection if the destination is a local drive or copy it if the destination is a network or removable drive. It is simplest to have the destination folder visible before you start to drag and drop, but you can cause a window to scroll up or down during the drag process or make a minimized window active by dragging the selection over the window's taskbar icon. Right-click drag the object and select an option from the shortcut menu displayed when you release the mouse button. Use the Edit \> Move to Folder/Copy to Folder commands. Use the Send To command from the main menu or shortcut menu to copy a file to a disk or send it by email. If a folder contains a file with the same name as the file being pasted, a confirmation dialog is shown: You can choose to overwrite the destination file, cancel the paste operation, or keep both files by renaming the one you are moving or copying (in Windows 8, choose the Compare info for both files option to do this). If doing this with several files, there is also a check box to choose the same option for all conflicts. Deleting Files and the Recycle Bin To delete a file using Explorer, select it then press DEL (or use the shortcut menu). Confirm the action using the prompts. If you accidentally delete a file or folder from a local hard disk, you can retrieve it from the Recycle Bin. A retrieved file will be restored to the location from which it was deleted. The size of the Recycle Bin is limited by default to 10% of the drive's capacity. If large numbers of files are deleted, those files that have been in the Recycle Bin the longest will be permanently deleted to make room for the newly deleted files. To recover a file, open the Recycle Bin , right-click the icon(s) to recover, and select Restore. If disk space is low, the Recycle Bin can be emptied (right-click the Recycle Bin icon and select Empty Recycle Bin from the shortcut menu). This process will permanently remove deleted files. From a security point of view, note that the data is not actually erased until that area of disk is overwritten by different data. Thirdparty utilities can recover files that have been "deleted" in this way. Other file "shredding" utilities are available to properly erase confidential data. You can set the amount of space to use on a per-drive basis or set one Recycle Bin for all local drives. You can also choose to suppress the use of delete confirmation dialogs. To set these options, right-click the Recycle Bin and select Properties. There is also an option not to use the Recycle Bin. If you want to delete a particular file without using the bin, hold down the Shift key as you delete it. The Recycle Bin works only for local hard drives, including USBconnected hard drives but not with flash memory thumb drives, removable media, or network folders. Files deleted from these locations are deleted permanently, though on a network, the administrator may be able to recover a file from the server itself. Selecting Multiple Files and Folders You can also perform actions on multiple files and folders. To do so, you need to be able to select the icons you want. There are various ways of doing this: Click and drag the mouse cursor around a block of files or select the first icon then hold SHIFT and click the last icon to select a block. You may want to sort the icons into a particular order first (see the "Searching for Folders and Files" topic below). Select the first icon then hold CTRL and select any other icons you want. Use SHIFT with the ARROW keys to select a block of files using the keyboard. Use CTRL with the ARROW keys to keep your existing selection, using the SPACEBAR to add icons to it. File Attributes and Permissions A file's name is just one of its attributes. Other attributes include the date the file was created, accessed, or modified, its size, its description, and the following markers, which can be enabled or disabled: Attribute Usage Read-only (R) Prevent changes being saved back to the file. The user will be prompted to create another file containing the modified data. Hidden (H) Specifies whether the file is visible in the default view (it is possible to adjust Windows to display hidden files and folders though). System (S) Specifies that the file should not be accessible to ordinary users. Archive (A) Shows whether a file has changed since the last backup. Files stored on an NTFS partition have extended attributes, including permissions, compression, and encryption. File Properties Dialog You can set some attributes manually using the file or folder's properties dialog. To open the properties dialog for a file or folder, right-click and select Properties. The properties for a folder will show the size of all the files in that folder (plus any subfolders). The properties for a file (or selection of multiple files) will show the file size. Note there is an actual file size and a size on disk shown. The size on disk is usually larger due to the way storage locations (or clusters) are created on the disk. You can also view file and folder information in the Details Pane in File Explorer or in Details view (see below). Folder and File Permissions To view, create, modify, or delete a file in a folder, you need the correct permissions on that folder. Permissions can also be applied to individual files. Administrators can obtain full permissions over any file, but standard users can generally only view and modify files stored either in their profile or in the public profile. If a user attempts to view or save a file with insufficient permissions to do so, Windows displays an Access Denied error message. Custom permissions can be configured for a file or folder using the Security tab in its properties dialog. Remember that permissions can only be configured if the file system is NTFS. FAT does not support permissions. To configure permissions, you first select the account to which the permissions apply. You can then set the appropriate permission level. In simple terms, the permissions available are as follows: Full control---allows the user to do anything with the object, including change its permissions and its owner. Modify---allows the user to do most things with an object but not to change its permissions or owner. Read/list/execute---allow the user to view the contents of a file or folder or start a program. Write---allows the user to read a file and change it, or create a file within a folder, but not to delete it. Windows Search enables you to locate files and information located in on your computer, within apps, such as email, or on the web. Search makes automatic use of file and folder properties (or metadata) and file contents. In Windows 10, the simplest way to search is to press the START key and type a search phrase. Files, programs, apps, messages, and web pages that match your search are displayed instantly: In Windows 10, the search box is located next to the Start button. You can type your search text straight into the box, or you can use vocal commands to initiate a search by using Windows Cortana, Windows 10's digital assistant. Type the required search string, and if necessary, click the Apps, Documents, or Web tab to filter results accordingly. File Explorer Search To search for files, you can also use File Explorer. The Explorer search box is located in the top-right corner of the window. Pressing F3 in Explorer activates the search box. You can open, rename, delete, move, and copy files from the search results as normal. If a basic search does not locate the file you want, you can add a filter to reduce the number of results: View, Group, and Filter Options In any folder, you can also use the view options to make finding a file or files easier. The view options set how large icons are, and you can use Details view to show information about each file in columns. The column headers allow you to sort files in ascending or descending order (or in other views you can right-click and select Sort By). The column headers or right-click menu also allow you to group and filter by the information in that field: Group---show icons in groups with dividers between them (for example, all files with names beginning "A," then all files beginning with "B," and so on). Filter---show only the files that match the selected criteria. The contents of libraries display an Arrange By option, allowing you to stack files in virtual folders according to the chosen field (Author or Type for instance). The Windows Search service indexes files and folders in the background. Indexing (and re-indexing) files can slow the computer down, so you may need to configure Indexing Options from Control Panel. File Types and Extensions It is worth knowing some of the extensions used to identify common file types. Word Processing Software The following file formats are often used by word processing software: txt---a text-only file with no "binary" file information linking the file to a particular software application. Any application can open a text file, but this file type cannot store any information about formatting or layout. rtf---Rich Text Format is an early "generic" file format for sharing documents between different word processing applications. It is capable of storing basic formatting information, such as font and paragraph formatting, and layout features, such as tables. odf---the Open Document Format is an XML-based specification with better support for the features of modern word processors than RTF. doc/docx---this format is the one used by Microsoft Word. The docx XMLbased format was introduced in Word 2007. Spreadsheet Software Microsoft's Excel spreadsheet software saves files with an xls or xlsx (Excel 2007 and up) file extension. Presentation Software Microsoft's PowerPoint presentation software saves files with a ppt or pptx (PowerPoint 2007 and up) file extension. PDF Viewers and Creators Adobe's Portable Document Format (PDF) is a file format for distributing documents. It is now an open standard, so different productivity applications can use it. For example, you could save a Microsoft Word document to PDF format and then open it in the Adobe Reader PDF viewer application. Most web browsers have plug-in PDF viewers. PDF was envisaged as a "final" format for the distribution of a published document. A PDF should look the same on-screen as it does when printed. It is possible to edit PDFs (using special applications) or to export a document from PDF to another format. In most cases though, it is important to keep a copy of the document in its "native" format. For example, having published a PDF from a Word document file, you would also save the latest changes to the Word file and keep it as the source file for any future changes. Image File Types DTP and graphic design applications (and most productivity software) can make use of images in digital file formats. A number of different image file formats have been developed for use in different scenarios: jpg/jpeg (Joint Pictures Expert Group)---this lossy compression format is the most widely used for photographic pictures. The lossy compression method relies on dithering the image to some extent (changing the color value of some pixels). The user can select a level of compression when saving the file, trading picture quality for reduced file size. A lossy compression method irreversibly discards some of the original data. This will, for example, reduce the quality of an image. Lossless compression is fully reversible because no information is discarded (it is just stored more efficiently). gif (Graphics Interchange Format)---this is an old lossless compression format. It only supports up to 8 bits per pixel, seriously limiting the available color palette. An 8-bit image can have up to 256 color values. Modern image formats support up to 24 bits per pixel, allowing a palette of millions of color values. tiff (Tagged Image File Format)---this is a popular format for exchanging images between editing applications. It can use lossless or JPEG compression. png (Portable Network Graphics)---this is a full-color (24-bit) lossless format designed to replace GIF. It also supports transparency. bmp---this is a Windows-only lossless format. It is not widely used due to its lack of compatibility with other operating systems. Video File Types mpg---this is an early MPEG (Motion Pictures Expert Group) standard for video files with lossy compression. mp4---the MPEG-4 standard audio/video file format. The format acts as a container for audio and video media streams (plus additional media, such as subtitles). A number of different encoding methods (or codecs) are available. One of the most widely used is H.264. flv---another container file format designed to deliver Flash Video. This is video created in the Adobe Flash developer tool. It can be viewed through the free Flash Player browser plug-in. Flash was once ubiquitous on the web but its use is declining since Apple refused to support it on the iPhone and iPad. The HTML5 web page coding language provides a standardsbased alternative to Flash. wmv (Windows Media Video)---a video container file format developed by Microsoft. It is well supported by media players and can also be used as the format for DVD and Blu-ray Discs. avi---a legacy Windows-only video format. It is a limited format with not much ongoing support. Audio File Types mp3---developed from MPEG, this remains one of the most popular formats for distributing music and is almost universally supported by media players. The only drawback is that it is a lossy compression format, which means that some of the audio information is discarded. aac (Advanced Audio Coding)---developed from MPEG as a successor to mp3. This format is also widely supported. m4a---this is an audio-only file format deriving from the MPEG-4 standards track. It usually uses AAC compression, though other methods are available (including lossless ones). flac (Free Lossless Audio Codec)---as the name suggests, this format achieves file size compression without discarding audio data. The only drawback is that it is not quite as widely supported by media players. wav---this is an early Windows audio file format. It is not widely supported by media players but may be used by audio editing applications. Executable Files An executable file is one that contains program code. Unlike a data file, program code can make changes to the computer system. Most operating systems enforce permissions to restrict the right to run executable code to administrator-level users. exe---this is the basic type of program file in Windows. msi---this is a Windows Installer file used to install and uninstall software applications under Windows. app---this is the equivalent of an exe file for macOS. bat/cmd/vbs/js/ps1---contains a sequence of commands either from the operating system's command interpreter or from a scripting language supported by the OS. A script is slightly different from program code in that it is not compiled into executable code. Instead, the script runs within an interpreter. Compression Formats Often, to send or store a file it needs to be compressed in some way, to reduce the amount of space it takes up on the storage media or the bandwidth required to send it over a network. There are a number of compression utilities and formats. zip---this format was developed for the PKZIP utility but is now supported "natively" by Windows, Mac OS X, and Linux. "Natively" means that the OS can create and extract files from the archive without having to install a third-party application. tar---this was originally a UNIX format for writing to magnetic tape (tape archive) but is still used with gzip compression (tgz or.tar.gz) as a compressed file format for UNIX, Linux, and macOS. A third-party utility is required to create and decompress tar files in Windows. rar---this proprietary format is used by the WinRAR compression program. 7z---this type of archive is created and opened using the open-source 7-Zip compression utility. gz---this type of archive is created and opened by the gzip utility, freely available for UNIX and Linux computers. A number of Windows third-party utilities can work with gzip-compressed files. iso---this is a file in one of the formats used by optical media. The main formats are ISO 9660 (used by CDs) and UDF (used by DVDs and Blu-Ray Discs). Many operating systems can mount an image file so that the contents can be read through the file browser. vhd/vmdk---these are disk image file formats used with Microsoft Hyper-V and VMware virtual machines respectively. A disk image is a file containing the contents of a hard disk, including separate partitions and file systems. Like an ISO, such a file can often be mounted within an OS so that the contents can be inspected via the file browser. dmg---this is a disk image file format used by Apple macOS.