The reasonable threat, and to his credit cipherpunk does encourage people not to paranoid, is that your old drive will be bought and connected to a new system in an ordinary manner.
As such, here is my opinion on what to do with your used hard drive to protect your data:
- Smash the drive, broadside, four times with a hammer, halfway between the center and the edge of the drive at each of the compass points.
Yes, this method means that you won't be able to sell your old drive on EBay, but it will keep your credit card numbers, diary entries, and goat pr0n out of the hands of others. In my opinion, losing ten bucks on the sale of a used hard drive is worth the peace of mind.