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.
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.
Disk images were originally used for backup and disk cloning of floppy disk media, where replication or storage of an exact structure was necessary and efficient.
On Macintosh computers running Mac OS X, disk images are now ubiquitous for software downloads. Typically, when a user initiates a download of a software package (often an application or printer/scanner driver), the web browser will download a compressed disk image file (.dmg suffix), which is saved automatically to the user's Downloads folder. Usually the disk image file will then automatically open, creating a mounted volume which appears on the user's Desktop - this volume contains the desired installer or other software. (If the disk image does not automatically open and mount, this can be initiated by double-clicking the disk image file.) Frequently, the installation is done by drag-and-drop, where the software is displayed beside an alias to the destination folder; the user completes the installation by dragging the software to the alias. Sometimes an installer package on the mounted volume will auto-launch (the OS prompts for an administrator password before proceeding with the installation). Once the installation is done, the mounted volume can be "ejected". The disk image file itself can then be deleted, or retained for future use.
Full article ▸