A friend had a old 10 lb sub woofer that was blown and we thought it would be fun to see if we could erase a hard drive with it. I've always heard about hard drives and their sensitivity to magnets and wondered if they are really that easy to erase.
We sat a modern (less than 2 yr old) hard drive on the magnet of the speaker, waved it around, and let it sit over night. It had zero effect on the hard drive. It booted right up every time. So the casing of the current hard drives can protect from a regular 10 lb magnet. I would be intrested to see the minimum amount of force it would take to zap a regular hard drive. I've read you would need a powerful alternating magnetic field to effect the drive at all. I guess you dont' have to worry about your hard drives around regular large magnets.
My guess on why it did not work is that it is one of those things that may have been true in the past (older drives with not much shielding) but it is not true today.