Hi,
first of all, consider the importance of your HDDs content.
If it could ruin your life and if "they're after you", go to your local electronics shop, buy a microwave oven and a large fire extinguisher. Go outside, put the harddisk into the oven, start it, stand back, enjoy the sight then kill the fire with your new bought extinguisher. Afterwards, retrieve the smoking remnants of your ex-HDD and use a sledge hammer to create an interesting puzzle for the NSA's geeks... :-)
If you're not paranoid enough for all this:
- Get a commercial disk wiper/cleaner and use at least a "medium safety level" - even a novice, provided someone gave him the equipment, can recover data up to at least 5 (!) repeated randomized wipes! Have you ever thought about the origin of all the "amateur s*x tapes" on the net...?
Sold HDDs can ruin your life and that of your loved ones. Take care.
(and I'm not THAT paranoid... btw: "that you're not paranoid doesn't mean they're not after you!")
Regards,
Peter