Wiping Entire Drive with dd Command
You cannot use the dd command to wipe a hard disk that is currently booted and running. Instead, you will need a bootable Linux distribution, such as Knoppix.
Boot your system to the Knoppix disk by choosing "Boot to CD ROM" during the boot process. Open up a root terminal and issue the following command:
dd if=/dev/zero of=/dev/sda bs=1M
Replace the /dev/sda with the proper mount point of your hard disk.
This will fill the hard disk (sda) with zeros (from /dev/zero), overwriting all the data with blocks of 1 MB in size.
With this method, it is theoretically possible to retrieve data with forensic investigation. If you want even more security, you can fill your hard disk with random numbers. This process takes longer than the zero method, but the data cannot be recovered by any method. Again, assuming that /dev/sda is the hard disk you want to wipe, the command is as follows:
dd if=/dev/urandom of=/dev/sda bs=1M