Shred
From Linuxintro
Shred destroys all data on a disk. The challenge is: If you have a magnetic disk and write zeroes all over the disk to destroy the data, there will still be a possibility left to read this data if you are willing to put in an enormous effort. shred first overwrites all data using random data, then zeroes the disk.
Example:
shred -n 1 -z -v /dev/sdx
(One pass to shred, one pass to zero)
