Mount a DD-created Disk Image

I have a lot of old disk images created with simple 'dd' commands. These images include the whole disk (all partitions) and are usually compressed. From time to time, I'll need to add files to those images or access data from them. Ideally, it's just quick to mount them to do that.

First, you need to decompress the image file. For gzip formats, I like to use pigz, since it's much faster on multi-core machines.

Now, you need to understand that you can only mount particular partitions. So, how do you access a particular partition in a single image file? You'll need to use a loopback offset.

Run fdisk to determine the partition table and get the data you'll need to calculate the offset.

# fdisk -l /path/to/IMG.dd

Disk IMG.dd: 120.0 GB, 120034123776 bytes
255 heads,63 sectors/track,14593 cylinders,tot 234441648 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0002beb4
 Device Boot      Start         End      Blocks   Id  System
IMG.dd1   *        2048   230336511   115167232   83  Linux
IMG.dd2       230338558   234440703     2051073    5  Extended
IMG.dd5       230338560   234440703     2051072   82  Linux swap

Now you need to calculate the offset. In my example above, I'm interested in partition #1, which fdisk reports to start at sector 2048. Since there are 512 bytes in each sector, as reported, multiply 512 X 2048 to get your offset value in bytes. In this case, it's 1048576. You'll use that number in the next step. Setup your loopback device offset, using the value you calculated above. I just used loop0, since it wasn't being used for anything on my system at the time. You should use whatever is available.

# losetup -o 1048576 /dev/loop0 /path/to/IMG.dd

Mount the loopback device.

# mount /dev/loop0 /mnt/img

Now you can use your filesystem as with any other file system you'd mount!

To unmount the file system, you'll need to unmount it like you normally would, but then you need to remove the loopback device.

# umount /mnt/img# losetup -d /dev/loop0