Using below dd command to backup an entire copy of a hard disk to another one, open ternimal or shell prompt and type the following command:

Code:
#dd if=/dev/hdc of=/dev/hdd
where,

if=/dev/hdc : input file
if=/dev/hdd : output file


2.backup hard disk as a image file

If you don’t want to bakcup disk to a disk, you can backup it as image file and save it in removeable storage device or Flash disk. then you can easy to use it in anywhere and can be quickly restored.

Code:
# dd if=/dev/hdc of=/hdc.image
Restore disk image
using the following dd command to restore a disk with the image file have backup before

Code:
#dd if=hdc.img of=/dev/hdc
3. Backup file from CDROM

dd command can generate an iso file from a source file read from CDROM.(insert the CD disk to CDROM)

Code:
#dd if=/dev/cdrom of=cdromfile.iso