Let us say, for the sake of simplicity, that you backup whole disks by using dd in Linux. Now let us assume that you need just a single file out of a backup and do not want to (or can not realistically) restore the whole backup to disk for the single file. You can not mount -o loop because the backup does not contain a single partition. “What to do” you ask? Well I will tell you, wary traveler.

1. Mount your raw disk image as a loopback device: losetup /dev/loop0 [path to image]
2. Run kpartx and: kpartx -va /dev/loop0
   This will add your partitions to /dev/mapper/loop0pX where each X is a different partition
3. You can now mount each partition: mount /dev/mapper/loop0pX /media/partition
   If you want to make sure you do not write anything to your backup simply mount it as read-only: mount -o ro /dev/mapper/loop0pX /media/partition

Once you are finished you will need to do your clean up.

1. Unmount: umount /media/partition
2. Remove the mapper devices: kpartx -d /dev/loop0
3. Remove the loopback device: losetup -d /dev/loop0

These commands may all need to be run as root. If loop0 reports that it is busy just pick another loop device (/dev/loop1, ect).