What Are .DMG Files?

DMG stands for disk image, and is a format commonly used to distribute files and applications among Apple computers. A DMG file is like a virtual DVD or hard drive. DMG files can be “mounted” on your Mac in order to work with their contents, or even burned to an actual physical disk.

Fig. 1 - A disk image mounted as a virtual drive.

In order to understand the concept of a DMG disk image, think of a storage volume such as a CD, DVD, hard drive, or external hard drive. A DMG file is like one of these devices in that it serves as a means to encapsulate documents, images, software, and other files. The difference between a DMG file and one of those devices is that with a DMG, there is no physical storage medium. There is only the DMG file, which can be written to a hard drive, burned to a CD or DVD, or sent over the Internet.

In order to work with the contents of a DMG file, you must mount the disk image to your system. This may sound daunting, however “mounting” a DMG file with Mac OS X is no more complicated than double-clicking it’s icon. The operating system will load the disk image and place a new drive icon on your desktop. You’ll also see that icon appear in the upper left-hand sidebar of the Finder. The icon will have the same name as the DMG file, and you will be able to explore it’s contents with the Finder.

Once you are done working with the contents of a DMG file, you will want to remove or “unmount” it from your system. Do this by opening the Finder and clicking the eject icon shown next to the virtual drive’s icon. You can also do this by going to your desktop, clicking once on it’s icon, and pressing CMD+E.