Just a tip from the old days of floppies.
It's a well known fact that, magnetic media like floppies and hard drives, don't stand magnetic interference.
So, to really destroy a hard drive's contents, open it up and put a strong magnet close to the copper plates.

This magnetic problem is why most hard disks have metal cases: to shield them from outside magnets.

mk6032
HELIX - computer forensics ... there.
Isn't there a possible conflict of trademarks with Helix Player from Real Media?

As for the old magnet:
I had a mate who swore that floppies were terribly unstable... and I was surprised. I'd had floppies last years in normal use. Whereas he would often find his data destroyed just inserting or removing the floppy from the drive. After I'd verified this effect, I took some effort to discover what was going on.

It turned out there was a small fridge magnet stuck inside the box just below the slot of the floppy drive. Unless you were very careful, you couldn't avoid passing a floppy over the magnet as you removed it.

Apparently an old flatmate had been playing a small joke...