You'd need a very strong magnet. A fridge magnet or speaker coil magnet would not work.
MD
How about microwaving the platters.
A simple magnet won't work to erase data if someone has tools to get it. What is needed is a very strong electromagnet running a degaussing routine... degaussing involves a frequent 180 degree field reversal with a declining strength. The key is to randomize all the bits many time over. A unidirectional magnet will put the bits in one direction, but by reading the relative strength of the bits, the data can be reconstructed with ease.