Run an old-timey tape bulk eraser across the drive case several times.
Frayed Knot OM! Most HD's are magnetically shielded. I even tried this myself just to bust this myth. I installed Windows on a Western Digital HD. After the system was up and running I pulled the HD and ran it over a Cart Bulk Eraser a quite a few times. The system still booted and there were no apparent problems during running.
Inside the HD there are sheets of magnetic media over and around the data chamber. If the drive is exposed to a magnetic field the magnetic field is simply absorbed and directed around the drive compartment instead of directly striking the data surface on the platter.
Disclaimer: That's not to say this can't be done on some of the many inexpensive drives. But my test on a common WD HD showed no damage at all.