using this method it will be virtually impossible to recover. note: this will take a lot of time to format depending on HD size.

1) Open Disk Utility (Applications -> Utilities -> Disk Utiliy.app)

2) Chose your hard disk from sidebar

3) Click the "Erase" tab

4) Click the "Security Options.." button

5) Chose option that best fit you

[ame]http://www.youtube.com/watch?v=clVravOAjb4[/ame]