A file with the .dmg extension is a type of Macintosh OS X disk image. This type of file can contain one or more file system volumes. When a dmg file is opened on a Mac, a disk volume icon will appear on the desktop as if an external drive was attached. This mounted volume appears to the Mac as any other disk drive volume. Changes to the volume change the dmg file instead of an actual disk drive.

The dmg format includes many options including various volume types (HFS, HFS+, HFSX, etc.), several compression options, encryption (password protection) and file spanning (segmenting). Compressed dmg files are generally read-only. Dmg files are often used for Mac software installations and general data exchange. Since internal compression is supported, this format is sometimes used instead of general compression archive formats like zip or sit.

While this format is generally exclusive to the Mac, our software TransMac can create, open and compress unencrypted dmg files on a PC in Windows. Even though TransMac can extract any file from a dmg and copy it to a PC drive, Windows will not be able to install or run Mac software. Only certain data files (image, video, text, html, sound, etc.) will be usable in Windows as long as you have applications that can open them.