[jimmyswimmy]

TFA said specifically that they could tell that the larger magnets were imposing a field inside the drive itself, and they could tell it was so because they could hear sounds coming from inside the drive. They presumed that the sounds were generated by deflection of the discs under the magnetic field. Therefore it is evident that the field is penetrating through the outer case.

They surmise that it was the higher coercive ty of modern drive plates that causes them to be resistive to reprogramming bits with a static magnetic field.