From http://www.linuxquestions.org/questions/linux-newbie-8/learn-the-dd-command-362506/

/quote

If you're concerned about spies taking the platters out of your hard drive, and scanning them using superconducting quantum-interference detectors, you can always add a "for" loop for US Government DoD approved secure hard disk erasure. Copy and paste the following two lines into a text editor.

Code:
#!/bin/bash
for n in `seq 7`; do dd if=/dev/urandom of=/dev/sda bs=8b conv=notrunc; done

Save the file as anti_scqid.

Code:
chmod +x anti_swqid

Don't run the program until you want to wipe the drive.

/end quote

I have yet to see any data be recoverable after just one pass. Of coarse I would by no means call my self a data recovery expert.

The process is not fast at all and can be quite time consuming but.... The drive also has to be repartitioned and formated because there is nothing left to find.