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Disk cloning is the process of making an image of a partition or an entire hard drive. This can be useful both for copying the drive to other computers and for backup/recovery purposes.The dd command is a simple, yet versatile and powerful tool. It can be used to copy from source to destination, block-by-block, regardless of their filesystem types or operating systems. A convenient method is to use dd from a live environment, as in a livecd.

Warning: As with any command of this type, you should be very cautious when using it; it can destroy data. Remember the order of input file (if=) and output file (of=) and do not reverse them! Always ensure that the destination drive or partition (of=) is of equal or greater size than the source (if=).

From physical disk /dev/sda, partition 1, to physical disk /dev/sdb, partition 1.

dd if=/dev/sda1 of=/dev/sdb1 bs=4096 conv=notrunc,noerror

If output file of (sdb1 in the example) does not exist, dd will start at the beginning of the disk and create it.

Cloning an entire hard disk

From physical disk /dev/sda to physical disk /dev/sdb

dd if=/dev/sda of=/dev/sdb bs=4096 conv=notrunc,noerror

This will clone the entire drive, including MBR (and therefore bootloader), all partitions, UUID’s, and data.

• notrunc or ‘do not truncate’ maintains data integrity by instructing dd not to truncate any data.
• noerror instructs dd to continue operation, ignoring all input errors. Default behavior for dd is to halt at any error.
• bs=4096 sets the block size to 4k, an optimal size for hard disk read/write efficiency and therefore, cloning speed.

Backing up the MBR

The MBR is stored in the the first 512 bytes of the disk. It consist of 3 parts:

1. The first 446 bytes contain the boot loader.
2. The next 64 bytes contain the partition table (4 entries of 16 bytes each, one entry for each primary partition).
3. The last 2 bytes contain an identifier

To save the MBR into the file “mbr.img”:

  # dd if=/dev/hda of=/mnt/sda1/mbr.img bs=512 count=1

To restore (be careful : this could destroy your existing partition table and with it access to all data on the disk):

  # dd if=/mnt/sda1/mbr.img of=/dev/hda

If you only want to restore the boot loader, but not the primary partition table entries, just restore the first 446 bytes of the MBR:

  # dd if=/mnt/sda1/mbr.img of=/dev/hda bs=446 count=1

To restore only the partition table, one must use

  # dd if=/mnt/sda1/mbr.img of=/dev/hda bs=1 skip=446 count=64

You can also get the MBR from a full dd disk image.

  #dd if=/path/to/disk.img of=/mnt/sda1/mbr.img bs=512 count=1

Create disk image

1. Boot from a liveCD or liveUSB.

2. Make sure no partitions are mounted from the source hard drive.

3. Mount the external HD

4. Backup the drive.

 # dd if=/dev/hda conv=sync,noerror bs=64K | gzip -c  > /mnt/sda1/hda.img.gz

5. Save extra information about the drive geometry necessary in order to interpret the partition table stored within the image. The most important of which is the cylinder size.

 # fdisk -l /dev/hda > /mnt/sda1/hda_fdisk.info

NOTE: You may wish to use a block size (bs=) that is equal to the amount of cache on the HD you are backing up. For example, bs=8192K works for an 8MB cache. The 64K mentioned in this article is better than the default bs=512 bytes, but it will run faster with a larger bs=.

Restore system

To restore your system:

 # gunzip -c /mnt/sda1/hda.img.gz | dd of=/dev/hda

badblocks is a Linux utility to check for bad sectors on a disk drive (A bad sector is a sector on a computer’s disk drive or flash memory that cannot be used due to permanent damage or an OS inability to successfully access it.). It creates a list of these sectors that can be used with other programs, like mkfs, so that they are not used in the future and thus do not cause corruption of data. It is part of the e2fsprogs project.

It can be a good idea to periodically check for bad blocks. This is done with the badblocks command. It outputs a list of the numbers of all bad blocks it can find. This list can be fed to fsck to be recorded in the filesystem data structures so that the operating system won’t try to use the bad blocks for storing data. The following example will show how this could be done.

From the terminal, type following command:

$ sudo badblocks -v /dev/hda1 > bad-blocks

The above command will generate the file bad-blocks in the current directory from where you are running this command.

Now, you can pass this file to the fsck command to record these bad blocks

$ sudo fsck -t ext3 -l bad-blocks /dev/hda1
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Check reference counts.
Pass 5: Checking group summary information.

/dev/hda1: ***** FILE SYSTEM WAS MODIFIED *****

/dev/hda1: 11/360 files, 63/1440 blocks

If badblocks reports a block that was already used, e2fsck will try to move the block to another place. If the block was really bad, not just marginal, the contents of the file may be corrupted.

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