PGP has a 'wipe' option that is Pretty Good (get it... ? sorry ) , and there is a UNIX utility named 'wipe' as well
that also seems to be quite good at scrubbing a disk. The problem as you now doubt are already aware of,
is that there is area on the disk called 'Slack' that is ordinarily not cleaned up during a typicall format.

The above mentioned utilities have worked well for me in a few circumstances when I had to have a completely
clean disk prior to re-use or storage. Note: PGP wipe as well as some others have been known to have
problems cleaning disks that are formatted with NTFS, there seems to be quite a bit of data that gets left
in the Slack area of the disk.