Can you erase data on your hard drive using a magnet?

Neodymium Magnets are strong magnets. In an experiment by one of such magnets' seller, we would expect a positive result. Well, K & J Magnetics did an experiment trying to erase the data of a harddisk.

With the hard drive running, we were not able to disrupt the contents of the drive at all. 100% of the files were completely intact and accurate. This result completely surprised us!

My own explanation is that the metal case of the harddisk, being magnetic, acts as a magnetic Faraday cage protecting the media inside the harddisk.