Would the hard drives of the computer in Cryptonomicon actually have been erased?

Question

In Cryptonomicon, we first see a computer taken out by an EMP. Afterwards, its hard drives are erased by a large electromagnet, as we learn later. Considering that the electromagnet was in the building hit by the EMP, could it still have been used against the computer's hard drives?

^{Note that this is not a duplicate of "Could a hard drive actually have been erased as described in Cryptonomicon?" (which I asked). That question asks if the mechanism is feasible, this question asks if it still would have worked considering the EMP.}

Answer 1

As I described in one of your other questions, an EMP works by generating a magnetic pulse which, in turn, induces a current in any conductor it passes through. (This is a rough approximation anyway.) (Interestingly, this would briefly "switch on" any electromagnet it happened to pass through if it wasn't already on to begin with.)

As I implied there, the amount of current generated in the target(s) is related to the amount of energy that went into the EMP. Electromagnets -- especially ones powerful enough to degauss a hard drive -- need to operate at very high currents. As a result, it's certainly possible that an EMP that fried sensitive electronics nearby, like the ICs in the computers or the cops' walkie-talkies, an act that takes very little current to pull off, would be woefully inadequate to harm the electromagnet even if it had no shielding of any kind.

Similarly, whatever is used to power the electromagnet must by necessity be able to generate (and, by extension, tolerate) the high current the electromagnet requires. It, too, is likely to survive an EMP only powerful enough to destroy relatively sensitives microchips. However, the power source might itself be computer controlled, or might have relatively delicate control circuits within it; in this case it would either be disabled, or would need to be shielded (unless it was located out of the effective range of the EMP). Being "tied to the grid" (i.e. using the power from the wall socket) would most likely not work at all, as the EMP, while most likely being inadequate to do any real harm, would almost certainly induce backcurrents in the electrical lines and result in breakers all up and down the grid being tripped, blacking out a good chunk of the area.

Answer 2

I am not intimately familiar with the specific events in the novel being referenced, however, it's certainly possible to utilize an electromagnet following an EMP attack.

However, you're going to either need a Faraday cage, or you're going to need to bury the electromagnet under ground (down a couple feet, possibly more depending on the wavelength of the EMP you're defending against). EMPs can damage electronics that are not connected to a power source.

Here's a video about using a Faraday cage to defend against EMP. (Note that a galvanized garbage can will function as a Faraday cage if you seal the lip with conductive tape; the lip serves as a slot antenna if you do not seal it, and the can provides almost no protection in that case.)