A disk image is a single file or storage device containing the complete contents and structure representing a data storage medium or device, such as a hard drive, tape drives, floppy disk, CD/DVD/BD and key drive, although an image of an optical disc may be referred to as an optical disc image. A disk image is usually created by creating a complete sector-by-sector copy of the source medium and thereby perfectly replicating the structure and contents of a storage device.
The disk image might be anywhere from several megabytes to several gigabytes in size, and will end in one of several extensions including .iso, .cue., .bin, or .img. For Macintosh computers, modern disk image files will end in .dmg.
A disk image is an excellent way to create backups of hard disk drives, as conventional backup programs can skip over files that are in use or are “inaccessible.” Backup programs might also copy data files but not program files. Moreover, a disk image copies the entire structure of the drive, including formatting. With a stored disk image on an external archive drive, if the main drive fails, the image can be copied to the replacement drive in minutes for a fully functioning system. It will be complete with personal preferences, tweaks, programs, files, and plugins.
Prior to disk imaging, the process of recreating a drive could take several hours, days, or even weeks. Drives had to be manually partitioned and formatted, and the operating system installed and configured. Programs were then reinstalled and reconfigured, followed by restoration of data files from backups. A disk image can take care of all of these time-consuming steps with a single, easy command. Disk images can also be used to back up CDs, DVDs, or floppy disks.
Disk images can be compressed to save space, or uncompressed — also known as “raw.” A compressed disk image saves room by using an algorithm that amounts to digital shorthand. When a compressed disk image is restored, the restore process includes expanding the shorthand to the original state as it is transferred to the new storage device.
Disk images burned to CDs can be used to evaluate operating systems without actually installing them. For example, Linux is commonly distributed as an .iso disk image that can be downloaded and burned for a live CD. Live CDs are bootable CDs that do not affect the host computer’s OS, but only utilize RAM. When finished with the live CD, the user can eject the disc and reboot the computer to the native operating system.
Some disk imaging utilities omit unused file space from source media, or compress the disk they represent to reduce storage requirements, though these are typically referred to as archive files, as they are not literally disk images.
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