As explained in my lengthy earlier reply, doing a 7 times overwrite is an unnecessary waste of time for a modern ATA drive.
Still more info about this: Although the 7 pass erase is often referred to as complaint with US DoD 5220.22-M, this is not exactly true. See Wikipedia's
National Industrial Security Program entry (the same reference linked to in
Mac OS X v10.5 or later: About Disk Utility's erase free space feature) for more about this. (The salient point is that DoD 5220.22-M does not actually specify any particular data sanitation algorithm.)
If you really want to hedge your bets & do a multi-pass erase, a US DoE algorithm 3-pass secure erase is a good choice. If you are using Snow Leopard, you can do this from the command line using diskutil. See
Programatically Secure Erasing Free Space | Krypted for the general method, but substitute "4" where it uses "0" for the level parameter & leave out "freespace" if you want to erase the entire device. For more about this, see the
manual page for diskutil for Mac OS X version 10.6.3.
Before trying this method, make sure you understand the basics of using the command line. The erase command is very powerful & if you do not know what you are doing you could erase the wrong disk, with no hope of recovering its data.
