If you have two hard disks in you machine and you want to be one and exact copy of the other one you can use the tool “dd” to do so.
Lets say you want /dev/sdb to be a clone of /dev/sda, then you can simply use the command:
dd if=/dev/sda of=/dev/sdb
Make sure, that /dev/sdb has at least the same size as /dev/sda!
How long does dd take?
Cloning a hard disk with dd can take several hours.
Assuming a average speed of 8 MB/s copying a disk with 120 MB would take about four hours.