@monorail unless you have really strong magnets it isn't, really, not on modern drives
the only thing that's really sensitive to magnetic fields would be floppy disks, and no one uses those anymore so it's not much of a concern (still, though, don't go putting your hard drive in an NMR machine)
@monorail there's actually quite a large magnet in the drives already in fact; the way the heads are moved is by using a permanent magnet linear actuator