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A disk image is a single file or storage device that holds a replica of all data on a storage medium or device, such as a hard drive, tape drive, CD, DVD, floppy disk or key drive. A disk image is usually created through a sector-by-sector replication of the original - or source - storage medium, including the structure (directories and folders) and contents (files).
Disk image is a noun and should be distinguished from disk cloning, which is a verb that describes the process of copying disk contents to another storage medium or image file.
Disk images were first used to back up and clone floppy disks. Exact disk structuring was necessary to have a similar copy of a floppy disk. Disk imaging became an efficient means of backing up data.
Today, online downloads of software applications are typically compressed disk images, with a “.dmg” suffix. Such disk images may be used to automatically create a mounted volume for easy installation of software.
Some software that is generally considered disk imaging software only backs up user files, rather than the disk structure, boot information or files locked up by the operating system (OS). These software tools do not create true disk images, which are required to be a perfect clone of the original storage medium.
Large organizations may need to clone many computers. However, copying all files or all storage media one by one is a waste of both time and resources. Thus, disk images are used to exactly replicate a fully prepared software environment.
Although typically referred to as archive files, some disk imaging utilities may omit space on the source media, or compress the data. Technically, these are not disk images.