An image backup is a complete copy of a hard disk or other media. The copy is complete in that it can be restored to an empty hard drive – for example, a replacement hard drive – the result being a hard drive containing everything from the original.
There are, of course, nuances to and even disagreement on the term’s meaning.
An image of a hard disk most commonly refers to a copy of all files, folders, and overhead information stored on the disk, including the information required to boot.
Another less common definition is that an image is a copy of every sector on the disk, including those not used, and their physical layout. This is more commonly referred to as a “clone“. See What’s the Difference Between a Clone and an Image? for a more exhaustive comparison and detailed definition.
The term image can be applied to either a partition or an entire hard disk. While making image backups of specific partitions can have value, only an image backup of the entire hard disk can be used to restore to a replacement hard disk.
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A disk image, in computing, is a computer file containing the contents and structure of a disk volume or of an entire data storage device, such as a hard disk drive, tape drive, floppy disk, optical disc, or USB flash drive. A disk image is usually made by creating a sector-by-sector copy of the source medium, thereby perfectly replicating the structure and contents of a storage device independent of the file system. Depending on the disk image format, a disk image may span one or more computer files.
The size of a disk image can be large because it contains the contents of an entire disk. To reduce storage requirements, if an imaging utility is filesystem-aware it can omit copying unused space, and it can compress the used space.