Have you ever lost data on a hard drive because of external magnetic fields?

For sure, if you get a degausser or other very strong field you will cause damage. But there are two strong magnets inside the case near the platters, so I'm not sure what kind of magnetic field is available in common domestic or office settings that might be harmful.

Magnetic field strength follows an inverse cube law, so an external magnet would have to be incredibly strong to alter data.

Magnetic field strength does not follow any specific dropoff law because it's a dipole.

The dropoff depends on how far apart the ends of the poles are. (i.e. from a distance the two poles blend together and average to zero), it also depends on the orientation - are the poles parallel or perpendicular?

True, but for the common case where the distance from the magnet is substantially greater than the distance between the poles, inverse cube is plenty good enough as an approximation.

Yep, possibly. It might make it more resistant to EM interference from the sun, or something, but not from a huge electromagnet.

Each bit will be smaller, so maybe they will be more vulnerable.