When you’ve read a disk with dd or ddrescue into an image file, one of the things you want to do is mount the partitions inside it. But, how does one do that? The answer is using losetup, with a bit of knowledge of partition offsets.

First, you have to map the disk image to a loopback device: 

# losetup /dev/loop/0 disk_image_file

Then use fdisk to print the partition table, displaying offsets in sectors: 

# fdisk -lu /dev/loop/0

For my disk, that results in: 

Disk /dev/sda: 250.0 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders, total 488397168 sectors
Units = sectors of 1 * 512 = 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *          63   482415884   241207911   fd  Linux raid autodetect
/dev/sda2       482415885   488392064     2988090   fd  Linux raid autodetect

You can see that the first partition begins at sector 63. Now we’re going to setup a new loop device, with the proper offset. A disk sector is 512 bytes long, so an offset of 63 sectors is 32256 bytes. Therefore, you setup the first partition with this command: 

# losetup -o 32256 /dev/loop/1 disk_image_file

Then you can mount that loop device: 

# mount /dev/loop/1 /mnt/mountpoint