Popular magnet isn't good solution. It's too weak to destroy all data. It will surely destroy some info but some data from hdd can be recovered. You want fast or slow method?
Fast:
Destroy it by hammer, use crane. Method is hard to do but no data can be revealed. Popular magnet's even if destroy data will leave some magnetic spectrum from which LE can reveal data. Cost's are very high but it's possible to them.
Slow:
Instal linux, format (slow), install windows, format (slow), instal linux, format (slow), install windows, use Eraser with 35 passes (Guttman method). It takes some time but you will be sure that none data can be revealed