You often have a hard disk drive that needs to be formatted such that all data in it gets destroyed. Here is a command that can erase all data from your HDD.
#dd if=/dev/zero/of=/dev/had bs=1M
This will formate your primary HDD(had) to a low level using dd. This will fill the entries disk with a sequence of zeros. In case you want to mix zeros and ones to fill the HDD, you can use the following command:
#dd if=/dev/urandom of=/dev/had bs=1M