File Extension DMG

For the avoidance of doubt .DMG files aren't actually "pictures" of Apple Mac software. Instead the .DMG file extension identifies Macintosh OS X (ie the latest version of the Mac operating system) disk image files. In the context of IT and computer software the phrase "disk image" means something other than what we might otherwise understand to be a photo of a disk, or an icon depicting a disk.

Indeed, in this context, "disk images" are similar (if still very distinct) to software copies and hard disk backups. The traditional copying of a CD, DVD, hard disk, or other media, would require that the computer read the file structure of the media and copy that across to somewhere else. However this only works if the PC has access to all those files: your PC won't (or at least not easily) be able to copy files that are protected, encrypted or otherwise not designed to be read by the system (such as the content of bootable disks).

Apple Computer

By contract disk imaging software takes a different approach, in that it duplicates the information actually on the CD, DVD, or other media, directly. By copying the individual bits of information from the source media the disk imaging software can create a perfect duplicate of the original content.

There are a range of applications of the principle of disk imaging. At first the technology was used simply to backup a hard drive, or create a clone of floppy media or a CD/DVD. However it is now a popular method for creating complete system backups which allow the system administrator to recover a damaged computer by loading the disk image of a previous configuration of that PC.

Similarly, in modern corporate environments, where there are a large number of identically configured PCs, systems administrators won't install all the software on every single computer, every single time. Instead they will configure one system correctly, then clone it (ie create a hard disk image) to every new PC they need (also called ghosting).

File Extension DMG