device image is a special kind of disk image ....

Launch it and drag the .dmg you wish to mount Once un-mounted, DMG Mounter will close itself

Here The Problem:
I decided to make a backup image using Disk Utility to an external drive, before wiping my drive and reinstalling from scratch.
Now I'm unable to mount the backup image, and the error "no mountable file systems" appears when I attempt to do so. I've tried booting up with the System disk and restoring the image to the drive, but that doesn't work. I've tried mounting the dmg using MacDrive in XP, but that didn't work. I've also tried using hdiutil to convert the dmg to an iso to mount in XP, and no luck there either. I've tried verifying and repairing the image file in Disk Utility, and neither worked.

Here The Answer:
A device image is a special kind of disk image that is an exact, block-level copy of the source volume and its filesystem, including features such as hard links and compressed system files. You can use device images to:

• Create a complete backup of your Mac. Device images support compression, so you can make the most efficient use of your storage, perhaps keeping multiple full backups on the same drive. They also support encryption, so that you can store your backup off-site (thus protecting against fire, theft, etc.) without worrying about anyone else accessing your sensitive files.
• Create a reusable image of a Mac, with everything installed and configured the way you want, then clone it onto multiple Macs, e.g. in a computer lab, so that they are all set up identically.

Device images have several advantages over other formats:
• Device images are generally faster to create than non-device images containing the same files.
• You can backup a bootable hard drive, then restore the backup, and the restored drive will be bootable.
• You can restore a device image to a hard drive without any special software. Just boot from the Mac OSX installation DVD that came with your Mac and choose Disk Utility from the Utilities menu.
• Device images efficiently store linked files, so they’re great for archiving old Time Machine backups.
• You can use Disk Utility to Verify Image... command to check that the backup device is not damaged and that you’ll be able to restore the files.

Since a device image is an exact copy of a drive, Disk Utility has to make sure that the drive contents don’t change while it’s making the image. In order to do this, it unmounts the drive, copies the data, and then remounts the drive. This sort of .dmg files fail to mount on your desktop with a "No mountable systems", here where it come DMG_Mounter !