The little, curved, shiny things are “hard drive magnets”, a metal bracket with Neodynium magnets attached. They are the strong, rare-earth permanent magnets that form half of the motor that moves the read-write head quickly across the surface of a hard disk platter. They are so powerful in fact, if you stick the flat ones to your fridge you’ll need to pry them off with a screwdriver. Most computer enthusiasts look at a picture like this and conclude right away that the data on that drive is toast.
There’s this old myth you can mess up or even erase the data on your hard drive by placing a whole bunch of fridge magnets on the outside of your case. To test this myth, I thought I might go one step further and cover a hard drive with really strong magnets to see what would happen. It seems sort of fitting that the strongest magnets I have are also the very magnets that make a hard drive work. I left them on for a day and a half.
The result? The data on the drive above was accessible after I pulled all the magnets off.
Since the drive pictured above was a little dodgy to begin with, I tried the procedure a second time with a different, known-good hard drive. As with the first one, there was no damage to the data at all. Not only did the second drive pass a CHKDSK, MD5SUMs of three ISO images on it were the same both before and after a sixteen hour period of being covered.
It would seem that hard drives are not as susceptible to magnetic fields as people think they are. This makes sense when you think about it a bit. Those powerful hard drive magnets are normally found inside a hard drive less than 2cm from the platters. Hard drives wouldn’t work at all if these magnets could erase the surface just by being close by.
The scientific reason why my ghetto drive erasing system doesn’t work is because for all their apparent magnetic power, the rare-earth hard drive magnets are simply not powerful enough to affect the particles on the hard drive platter when sitting on top of its cover. The coercivity of the magnetic material on a hard drive platter is very high, around two thousand Oersteds, and even though rare-earth magnets can have five times that level of coercivity, mine aren’t nearly so strong. Since there is a bit of space between the drive’s top cover and the surface of the platter, the affect of their magnetic field is even less. I could have a hundred of them piled on for a week and it wouldn’t make any difference.
So is it possible to mess up the data on a hard drive with permanent magnets? Sure, but not with the little ones you tend to find just lying around. Certainly not with fridge magnets. A big rare-earth one would probably do the trick, just as a bulk electromagnetic media degausser would. If you really, permanently want to wipe your hard drive, you could always check out the GuardDog prototype. It not only erases a drive using a strong magnet, it destroys it.