Research EMP effective range .
You will find that during Nuke Tests (MANY if not all) were recorded using standard video cameras, none of the sensitive CCD devices were destroyed - not even a single pixel.
Furthermore, trials for a device that would disable a vehicle were so ineffective (1 out of 10 kill rate) when jettisoned underneath the moving vehicle at range of only 1-2 ft…
Basically unless a mass coronal ejection occurs, nothing man made is very effective.
Considering "Effective Radiated Power" (ERP to radiofags) the exponential degradation of the signal in all directions, the height would widen the area, but the intensity would exponentially degrade.
So the only thing I would be seriously worried about is a mass coronal ejection (space/sky event). This is what the military specs protects against, along with magnetic disturbances.
A hard drive is encased in ALUMINUM, and uses magnetic disc. EMP would/could kill the pcb only, not the data on the disc.
Keeping drives in a firesafe is a good idea, a USB drive works inside the vault when a cable is ran thru the vault. This is mostly for fire protection, will help deter against theft also.
But if plugged in, surges are unpredictable, so I personally use two- and switch them out occasionally, you know- backup…