It'll do more than that. A hard disk is a magnetic platter. The "bits" are defined by the magnetic orientation of tiny portions of the platter. Running a strong magnet over it will flip all the bits in the same direction, like doing a write of all zeroes or ones. It's certainly the fastest form of data wiping if you do it right, so that may be your best option when the door is getting kicked in. DBAN takes 5-10 hours on an average hard drive to do 3 writes. A magnet takes a few seconds. Correct. It would be a huge privacy leak if it saved your browsing history to disk. If you save bookmarks or passwords, those will be saved to disk, though. Tor isn't illegal. I don't think you could be convicted of a specific crime like attempting to purchase a controlled substance, just by having Tor on your computer.