You have been watching too many TV
You have been watching too many TV shows and Movies.
First, you can't erase the platters of a hard drive from outside the drive using any normally avaialbe source of magnetic fields (if you have acces to an MRI machine, well, then .... maybe). The magnetic field strength required AT THE PLATTER to erase the platters is VERY high, and this strength decreases by the inverse square law. The heads are VERY close to the drive, within 20 one-millionth's of an inch. But by the time you get to even just a half-inch it's just about impossible ANY normally available souce of magnetic fields.
Second, if you could do that, it would not only destroy the data, it would destroy the drive (which you don't care about). The reason is that in addition to erasing the data, if you could "bulk erase" the platters, you would also erase the drive's servo information. The drive can't operate without this, and it can't be recreated in the field, so you have destroyed the drive.
Third, however, I take issue with the callousness of all of the replies (yours included) that basically say to destroy / discard the drive as worthless. The drives do have value, and it's wasteful and environmentally unfriendly to pitch a perfectly good drive when a simple wipe of the drive will resolve all legitimate security issues that a normal person would reasonably have. I teach computing at a local college, and we can always use discarded drives. Even defective drives make good teaching tools when we take the covers off and let students have them as we try to explain concepts like tracks and secotors and cylinders and heads and platters and so on.