Well, bugger all effect, I would have thought. Stainless steel (the case) is still steel, and although it has reduced ferro magnetic properties, it still will accept a field and route it through it's "magnetic domains" around the circuitry/disks.

I suspect you would probably need an field strength somewhere close to that in the toroid field created by the European JET to create a favourable magnetic bottle sufficient to contain nuclear fusion before a shielded drive with a 5 micron magneto-resistive head would be affected.

A beer is in order, methinks....