[thatsnotnice]

Just get a set of torx (or tamper resistant torx) drivers and take the drive apart.

You'll get a couple of hellacious refrigerator magnets and the platter assembly makes a nice paperweight for your desk (especially if you can get hold of the old Seagate 1.2M 5.25 inch full-height SCSI drives).

[Flying Circus]

A simple magnet is not sufficient to erase data. It will change the direction of the magnetic moment of the disk, but the with the right tools individual bits still have enough memory to reconstruct the original data. What you need is a degauss tool which will cycle the magnetic field direction a hundred times over and completely randomizes the bits.