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 installation packages 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 dmg files on a Windows PC. Even though TransMac can open and extract files from a dmg image, only certain data files (image, video, text, html, sound, etc.) will be usable on a PC. Windows will not be able to install or run Mac software. TransMac is also capable of burning a dmg to CD/DVD.

Burning dmg files
Most .dmg files are compressed and must be expanded before burning to CD/DVD. The expansion process is popularly called "convert to ISO" on the web (due to a program called dmg2iso), but that is not technically correct. ISO (or ISO 9660) is an older file system format used on CDs. Actually converting a dmg to an ISO 9660 image would not make sense since it would remove all of the Mac specific data. Burning dmg files is a two step process - 1)Expand the dmg and 2)Burn the expanded dmg (as an image not a file). Some burning programs may require renaming the dmg with a .iso extension before it will be recognized.

Burn a dmg file to a CD/DVD using TransMac:
1) Double-click the dmg or select Open Disk Image from the File menu.
2) Right-click the dmg in the left treeview pane and select Expand. Make sure you have room for the expanded image on your disk drive since it will be larger than the original.
3) After expanding, TransMac asks to open the expanded ISO dmg. Select Yes.
4) Right-click the expanded dmg and select Burn to CD/DVD. Select the CD/DVD drive and click OK.