The DMG and IMG file extensions are both primarily disk image files used on Apple computers, used for packaging and distribution of software. The IMG file extension was the primary format of its kind for versions of Macintosh’s operating system up to OS 9. With the release of OS X in 2001, DMG replaced IMG as the primary disk image file type.
DMG vs. IMG
- The primary functional difference between the DMG and IMG file type is IMG files would split some of their information into a resource fork found separately on the drive. Without the resource fork, which would often be lost when the IMG file was sent around online, the IMG would not be able to be opened, thus making file sharing more difficult with the IMG format.
DMG Format on OS X
- When Apple switched to using the DMG format, it allowed the information previously stored in the data fork with IMG files to be also included in the disk image file. The D in DMG in this case stands for data, as files of this type can be shared and launched without any additional files on the system. DMG files can also be used as a compression format like a standard ZIP file, allowing many files to be contained in one easy-to-share archive.
Disk Image Mounting
- Both DMG and IMG files can still be burned to a disc using Apple’s Disk Utility program to create bootable discs, and both can be converted into ISO files, which are images of complete CDs or DVDs. As well, software is available to transform one file to the other component, assuming that you have all the necessary files and the image itself is a bootable installer. There are as well, though, IMG files that are simply graphic image files. You will be able to tell an IMG is a disc image if its icon looks like a small drive with its upper left corner folded down.
Prevalence of DMG
- Because DMG files are the most recent format supported by OS X, they are the more common and popular version of disk image files for usage. Often software that you buy online will come packaged as a DMG file that you can install to your machine directly by simply clicking it and following OS X’s automated installation process. When the DMG is used as a ZIP archive, it will unzip the archive when the DMG is double-clicked, placing a folder of the archive’s contents on your desktop.
