Maybe magnetism could work. HDD platters take very strong fields to change magnetization these days, so erasing the data with the positioning magnet(s) seems avoidable.
Also, you could imagine a magnetic bearing whose field strength rapidly diminishes as you move away from it, due to cancellation, like a Halbach array [0]. I wonder if there's a simple geometric configuration for a bearing that has this property, as well as the critical property of passively stable levitation, maybe based on diamagnetic or superconducting materials [1].
The raw words written to the drive are actually re-encoded into slightly larger codewords with nice properties like not having too many zero or one bits in a row, and error detection/correction.
Plus I think that the 0/1 bits are not encoded as "no magnetism"/"some magnetism", but instead as "north magnetism"/"south magnetism" since magnetic fields have a direction.
And I don't think the magnetic fields on the platters have any appreciable effect on the head besides the electromagnetic effects at the sensor.
Essentially the same problem as with fiber optics. The data can't be recovered unless there are frequent bit transitions. In that case the data is transformed to ensure illegal patterns cannot occur.
