A Disk Image is a mirror copy of your entire logical drive or physical device stored as set of files. It may be a good idea to create a Disk Image for a drive containing deleted files that you want to recover, if you have enough space on another drive.
"Why should I create a Disk Image on a drive that holds my deleted files?"
If you run into difficulty, or do something wrong while attempting to recover deleted files (for example, by recovering them onto the originating drive instead of a different drive, thereby destroying their contents), you will be able to recover these deleted files and folders from the Disk Image that you have wisely created.
Disk Image consists of configuration file (with an extension .DIM) and set of files having extensions .000, .001, .002 and so on. The size of each file is 2 Gb to accommodate most file systems (FAT16 and FAT32 file systems do not support file sizes larger than 2 Gb and 4 Gb respectively).