The strength of the magnetic field to revease a domain on a plater is very strong. Ptimarily this is do to distance. The heads are VERY close to the plater and the strenght of a magnetic field is inversly proportional to the square of distance. Even using a magnetic screw diver for puting the screws into the side of a HDD would have absolutly no evect on HDD platers, Bear in mind there is a magnetic shiled on HDD. This can be verified by leaving the magnetic screw driver siting on top of a 3.5 in Floppy disk and check for corrupted files.
jsc - your approach is just as good.
Old story. When I was in junior high they demonstrated magnitizing a nail with a couple of "D" cells and a coil of wire. I thought, great let me try this at home with a larger voltage - 120 VAC. Wound my coil around a large nail and plugged it into an outlet. Needless to say my dad was not to happy replacing a AC outlet and the fuse. Live and Learn.
@nd story. My wife will never let me forget. Before my entry into electronics. !957 Mercury with push button transmition. Used the lock-out relay as a anty theft device. The wire came loose while driving (ground lead. Told my wife to stick back on ground. She asked will I get shocked - I responded - NO (like 12 volts). She did as instructed and promply got hit by several thousand volts. she was not pleased!