# 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 partition table from being written to disk. You can manually restore the table.