Hi Guys,
Well I don't have an MSc in Physics, but I do have a 1st Class Honours in Theoretical Physics albeit 24 Years ago ....
My recollections are that the EMP spike is so intense even more so than lightening that if any substance is remotely conductive, the chances are EMP will have an effect (normally bad) on it. The better the conductivity obviously the higher the current induced in it and the more potential for damage is possible.
However ... Never forget the protective effect of the Faraday Cage. If you hear some EMP is heading your way ... quickly shut down your PC, build yourself a box out of the smallest wire mesh you can find and make sure you securely earth it. Put the computer inside it and your whole computer will be fine. The key is the fact that no conductor touches the computer or breaks the barrier.
Having a metal case around a HDD is not protective enough, you have conductors breaking the barrier so yes your HDD will in all likelyhood be fried/wiped.
Mind you if we have to worry about EMP, the last thing I'd be concerned about is my bloomin' HDD ...
