How big a magnet do I need to wipe my portable HDD?
I doubt you'll be able to destroy your data with a magnet without opening the drive up. Inside of the drive are a pair of neodymium magnets (similarly strong to the ones you have, but just not as large), within centimeters of the platter. The drives are well shielded, so it is near (if not entirely) impossible to wipe them by waving a magnet around them.

What article are you referring to?