In every hard disk there is something like a super magnet, which is stuck on a plate. But there is almost no magnetic field beneath the plate. How does the metallic plate reduxe the field?
Those rare earth magnets aren't just sitting in there. They're part of the voice coil head positioning system (vs. using a stepper like a floppy, giving the HDD a lot faster seek time). They're in pairs and the magnets have their field carefully contained, basically "conducting" it via the ferrous frame onto which they are mounted. There's isn't much shielding needed to contain any stray field, for there isn't much stray field to contain. Kinda like focusing a lamp using an lens.