How to use DD to clone a partition off a disk image?

Question

I have a disk image, it's a "whole" disk image, e.g., contains multiple partitions, and I want to clone just one of them (not the first one..) onto a partition on an external drive with multiple partitions on it (I'm also not cloning it onto the first partition of the disk...)

FDisk'ing the image gives this:

# fdisk -l 2013-02-09-wheezy-raspbian.img 

Disk 2013-02-09-wheezy-raspbian.img: 1939 MB, 1939865600 bytes
255 heads, 63 sectors/track, 235 cylinders, total 3788800 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: 0x00014d34

                         Device Boot      Start         End      Blocks   Id  System
2013-02-09-wheezy-raspbian.img1            8192      122879       57344    c  W95 FAT32 (LBA)
2013-02-09-wheezy-raspbian.img2          122880     3788799     1832960   83  Linux
#

and the block device looks like this:

# fdisk -l /dev/sdc

Disk /dev/sdc: 8014 MB, 8014266368 bytes
247 heads, 62 sectors/track, 1022 cylinders, total 15652864 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: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1            2048      131071       64512    e  W95 FAT16 (LBA)
/dev/sdc2          131072    15652863     7760896   83  Linux
#

I want the second partition of the image to replace the second partition of the block device. Don't worry about the trailing corrupted free space, I'll use GParted to clean that up, and I need it for something else anyway.

Answer 1

# losetup --find --show --partscan --read-only 2013-02-09-wheezy-raspbian.img
/dev/loop7
# dd if=/dev/loop7p2 of=/dev/narnia bs=1M

If --partscan doesn't work, you can also use one of:

# partx -a /dev/loop7
# kpartx /dev/loop7

or similar partition mapping solutions.

You should probably mount it first just to see if it's the right thing or what.

Of course you can also read the fdisk output and give dd the skip=131072 or whatever directly, i.e. make it skip that many blocks of input so it starts reading at where the partition is located; but it's nicer to see actual partitions with a loop device.

Answer 2

Something like this should work:

dd if=/path/to/2013-02-09-wheezy-raspbian.img2 of=/dev/sdc2 bs=4096 conv=notrunc,noerror

if= sets the image you want to clone

of= sets the target partition where you want to clone the image

Answer 3

1. Mount the image as a loopback device: losetup -fP yourImage.img
2. You may now access each partition of the image directly. If you must use dd, you can do dd if=/dev/loop0p1 of=image.img, but I think that you would like cat better. In a root shell, simply do cat /dev/loop0p1 > image.img. Both examples assume that you want the first partition, and the loopback device you were assigned was /dev/loop0
3. Once you are done with the loopback device, you can detach it with losetup -d /dev/loop0. Once again, this assumes that /dev/loop0 is your loopback device.

Answer 4

You could do:

sudo partx --add -v 2013-02-09-wheezy-raspbian.img 
# time sudo dd if=/dev/loop0p1 |pv -s 80M |sudo dd of=/dev/sdc1
time sudo dd if=/dev/loop0p2 |pv -s 4G |sudo dd of=/dev/sdc2
sudo partx --delete -v /dev/loop0

Answer 5

You can copy just the second partition to a separate image with this

dd if=2013-02-09-wheezy-raspbian.img of=second.img bs=512 skip=122880

and after that you can copy it in place of the partition on the disk

dd if=second.img of=/dev/sdc2 bs=512

or you can even do it in one command

dd if=2013-02-09-wheezy-raspbian.img of=/dev/sdc2 bs=512 skip=122880

Just make sure that you unmount /dev/sdc2 before you write to it.

Answer 6

Here is the easiest solution :

dd skip=122880 count=3788799 status=progress if=2013-02-09-wheezy-raspbian.img  of=secondpartition.img

as it shows in the output of fdisk, fdisk is using 512 bytes as the default block size:

# fdisk -l 2013-02-09-wheezy-raspbian.img 

Disk 2013-02-09-wheezy-raspbian.img: 1939 MB, 1939865600 bytes
255 heads, 63 sectors/track, 235 cylinders, total 3788800 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: 0x00014d34

and as it says in the dd command help, 512 bytes is the default value for the switches "bs", "cbs", "ibs" and "obs"

      bs=BYTES        read and write up to BYTES bytes at a time (default: 512);
                  overrides ibs and obs
  cbs=BYTES       convert BYTES bytes at a time
  conv=CONVS      convert the file as per the comma separated symbol list
  count=N         copy only N input blocks
  ibs=BYTES       read up to BYTES bytes at a time (default: 512)
  if=FILE         read from FILE instead of stdin
  iflag=FLAGS     read as per the comma separated symbol list
  obs=BYTES       write BYTES bytes at a time (default: 512)
  of=FILE         write to FILE instead of stdout
  oflag=FLAGS     write as per the comma separated symbol list
  seek=N          skip N obs-sized blocks at start of output
  skip=N          skip N ibs-sized blocks at start of input
  status=LEVEL    The LEVEL of information to print to stderr;
                  'none' suppresses everything but error messages,
                  'noxfer' suppresses the final transfer statistics,
                  'progress' shows periodic transfer statistics

to confirm that you can use dd itself to create a one block file like this:

dd if=/dev/urandom of=test count=1

The file size should be 512 bytes.

So all you have to do to create an image of a partition or a group of partitions is to get the number of sectors for the start of the partition or first partition from the output of fdisk and use it as the value of the dd parameter "skip" and then the number of sectors for the End of the partition or last partition and set it as the value for the dd parameter "count".