According to this article, I need a really big magnet to wipe the data off my HDD. Thing is, I don't know how strong a "laboratory degausser" is, so I'll ask it in my own terms: I have a pair of magnets that would crush my fingers if I let them close on my hand. Is that likely to be strong enough to wipe the data on my damaged portable HDD before I let the repair guys fix the dinky connector between casing and drive proper? It's a sleek WD Passport that I can't open up myself without ruining the aesthetics. Don't care if they give me a new one, do care if someone gets to poke around the data in the old.
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?
If someone will die or have their life ruined if the drive contents are discovered, you need a big enough magnet to pulverize the drive when forcefully dropped onto it. Multiple, judicious applications of a magnetized sledge-hammer should serve nicely.
OTOH, if you just need to keep out the curious, a wipe and reformat should be more than adequate.
In addition to what Shamtam said about how waving a magnet around the outside of the case could be ineffective, it can do damage to other things, especially if you do this while power is applied. A strong but changing magnetic field (like that caused by waving a strong magnet) will induce current and voltage in anything conductive that happens to be at the right orientation. It doesn't take a lot of current or voltage in the wrong place to do bad things, like making a chip latch up.
Then there is also purely mechanical damage. Some things in this device are going to be magnetic, and a magnetic field is going to put a force on them. A strong field may make such a strong force that things get bent, metal touches something conductive it isn't meant to etc.
All around, this is a really bad idea.
If you want to erase a drive, I'd recommend that you run
dd if=/dev/zero of=/dev/hdX. Not only would it be difficult to get powerful enough magnets, you would probably damage the drive itself by degaussing its firmware. You could also just delete all the partitions on it...I doubt the repair guys would go through that much trouble to find out what you have on that drive.
Of course, if you can't access the disk because its broken, you can't just do a simple wipe or reformat. If the data on it is important to keep private, destroy the disk. Drilling a hole through the platter is fine to stop most nonprofessionals. If its important, don't send it for repair.