Linux DD Command

Backup an entire hard disk using dd command

The ‘ dd ‘ command is one of the original Unix utilities and should be in everyone’s tool box. It can strip headers, extract parts of binary files and write into the middle of floppy disks; it is used by the Linux kernel Makefiles to make boot images. It can be used to copy and convert magnetic tape formats, convert between ASCII and EBCDIC, swap bytes, and force to upper and lowercase.
For blocked I/O, the dd command has no competition in the standard tool set. One could write a custom utility to do specific I/O or formatting but, as dd is already available almost everywhere, it makes sense to use it.
Like most well-behaved commands, dd reads from its standard input and writes to its standard output, unless a command line specification has been given. This allows dd to be used in pipes, and remotely with the rsh remote shell command.
Unlike most commands, dd uses a keyword=value format for its parameters. This was reputedly modeled after IBM System/360 JCL, which had an elaborate DD ‘Dataset Definition’ specification for I/O devices. A complete listing of all keywords is available from GNU dd with
# dd –help

For more options check dd man page
Using dd you can create backups of an entire harddisk or just a parts of it. This is also usefull to quickly copy installations to similar machines. It will only work on disks that are exactly the same in disk geometry, meaning they have to the same model from the same brand.

full hard disk copy
dd if=/dev/hdx of=/dev/hdy
dd if=/dev/hdx of=/path/to/image
dd if=/dev/hdx | gzip > /path/to/image.gz
Hdx could be hda, hdb etc. In the second example gzip is used to compress the image if it is really just a backup.

Restore Backup of hard disk copy
dd if=/path/to/image of=/dev/hdx
gzip -dc /path/to/image.gz | dd of=/dev/hdx

MBR backup
In order to backup only the first few bytes containing the MBR and the partition table you can use dd as well.
dd if=/dev/hdx of=/path/to/image count=1 bs=512
MBR restore
dd if=/path/to/image of=/dev/hdx

Add “count=1 bs=446” to exclude the part ition table from being written to disk. You can manually restore the table.


UNIX / Linux: Copy Master Boot Record (MBR)

How do I copy MBR from one hard disk to another hard disk under Debian Linux?
To copy MBR simply use the dd command. dd command works under all Linux distros and other UNIX like operating systems too. A master boot record (MBR) is the 512-byte boot sector that is the first sector of a partitioned data storage device of a hard disk.

MBR Total Size

446 + 64 + 2 = 512


  • 446 bytes – Bootstrap.
  • 64 bytes – Partition table.
  • 2 bytes – Signature.

512 vs 446 Bytes

  • Use 446 bytes to overwrite or restore your /dev/XYZ MBR boot code only with the contents of $mbr.backup.file.
  • Use 512 bytes to overwrite or restore your /dev/XYZ the full MBR (which contains both boot code and the drive’s partition table) with the contents of $mbr.backup.file.

dd command to copy MBR (identically sized partitions only)

Type dd command as follows:
dd if=/dev/sda of=/dev/sdb bs=512 count=1
Above command will copy 512 bytes (MBR) from sda to sdb disk. This will only work if both discs have identically sized partitions.

dd command for two discs with different size partitions

# dd if=/dev/sda of=/tmp/mbrsda.bak bs=512 count=1
Now to restore the image to any sdb:
# dd if=/tmp/mbrsda.bak of=/dev/sdb bs=446 count=1
The above commands will preserve the partitioning schema.

Linux sfdisk Command Example

Linux sfdisk command can make a backup of the primary and extended partition table as follows. It creates a file that can be read in a text editor, or this file can be used by sfdisk to restore the primary/extended partition table. To back up the partition table /dev/sda, enter:
# sfdisk -d /dev/sda > /tmp/sda.bak
To restore, enter:
# sfdisk /dev/sda < /tmp/sda.bak
The above command will restore extended partitions.

Task: Backup MBR and Extended Partitions Schema

Backup /dev/sda MBR, enter:
# dd if=/dev/sda of=/tmp/backup-sda.mbr bs=512 count=1
Next, backup entries of the extended partitions:
# sfdisk -d /dev/sda > /tmp/backup-sda.sfdisk
Copy /tmp/backup-sda.sfdisk and /tmp/backup-sda.mbr to USB pen or somewhere else safe over the network based nas server.

Task: Restore MBR and Extended Partitions Schema

To restore the MBR and the extended partitions copy backup files from backup media and enter:
# dd if=backup-sda.mbr of=/dev/sda
# sfdisk /dev/sda < backup-sda.sfdisk



I overlooked this before. The steps you tried and the link I mentioned both seem to be lacking the file system formatting. After creating /dev/sdc1 using fdisk, you will need to format the file system before it will be usable.

mkfs.vfat -F 32 /dev/sdc1

After this, execute dd if=boot_image of=/dev/sdc1 and it should work.



Mounting a raw partition file made with dd or dd_rescue in Linux

This situation might not affect everyone, but it struck me today and left me scratching my head. Consider a situation where you need to clone one drive to another with dd or when a hard drive is failing badly and you use dd_rescue to salvage whatever data you can.

Let’s say you cloned data from a drive using something like this:

# dd if=/dev/sda of=/mnt/nfs/backup/harddrive.img

Once that’s finished, you should end up with your partition table as well as the grub data from the MBR in your image file. If you run file against the image file you made, you should see something like this:

# file harddrive.img
harddrive.img: x86 boot sector; GRand Unified Bootloader, stage1 version 0x3, stage2 
address 0x2000, stage2 segment 0x200, GRUB version 0.97; partition 1: ID=0x83, 
active, starthead 1, startsector 63, 33640047 sectors, code offset 0x48

What if you want to pull some files from this image without writing it out to another disk? Mounting it like a loop file isn’t going to work:

# mount harddrive /mnt/temp
mount: you must specify the filesystem type

The key is to mount the file with an offset specified. In the output from file, there is a particular portion of the output that will help you:

... startsector 63 ...

This means that the filesystem itself starts on sector 63. You can also view this with fdisk -l:

# fdisk -l harddrive.img
                    Device Boot      Start         End      Blocks   Id  System
harddrive.img                *          63    33640109    16820023+  83  Linux

Since we need to scoot 63 sectors ahead, and each sector is 512 bytes long, we need to use an offset of 32,256 bytes. Fire up the mount command and you’ll be on your way:

# mount -o ro,loop,offset=32256 harddrive.img /mnt/loop
# mount | grep harddrive.img
/root/harddrive.img on /mnt/loop type ext3 (ro,loop=/dev/loop1,offset=32256)

If you made this image under duress (due to a failing drive or other emergency), you might have to check and repair the filesystem first. Doing that is easy if you make a loop device:

# losetup --offset 32256 /dev/loop2 harddrive.img
# fsck /dev/loop2

Once that’s complete, you can save some time and mount the loop device directly:

# mount /dev/loop2 /mnt/loop

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s