Try: sudo shred -n2 -v /dev/sdx where sdx is your hard drive or partition
(will be followed by partition number such as sda1); n2 is the number of
shreds. More shreds is better but takes longer. You can also try: sudo dd
if=/dev/zero of=/dev/sdx (with sdx being same as above). In this case it
restores the drive to zero or factory state. I have never tried this as
shred works for me.