DMG files are files that are treated like actual hard disks by the Macintosh OS X operating system. These files can be created with Disk Copy, burned to a CD or even mounted as a normal volume on Macintosh computers. They often contain raw blocks of data that are usually compressed for easier file management.
Some .dmg files may be encrypted in addition to being compressed, for added file security. These files are often used by software installers that download applications from the Internet to a user's computer.
The Oracle export and import utility, used by Oracle databases, also uses the .dmg file extension for database dump files that are stored a binary file format.
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If you have additional information about the DMG file format or software that uses files with the DMG suffix, please do get in touch - we would love hearing from you.