Just as an offtopic aside removing the disks from a hard drive effectively destroys the data. One surface has a servo track and the data tracks on the remaining surfaces are referenced to that servo track. It's possible someone could try and manually align the data tracks to a different servo track but would be very labor intensive.
For the vigilant you could also run the magnet over the surface or bend the disk - even a slight amount will render it useless. Or for tin foil hat crowd you could CIA it and drill holes in the disk or scrape the coating off the disk.