Kind of a silly question, but I'm genuinely curious: (in part because I recently saw several questions that refer to "nuking" a hard drive...) what would a microwave DO to an HDD? Would it erase data on there, or at least corrupt it enough that it's difficult to recover?
In theory, no.
The outer shell is a Faraday Cage, so the radiation from the microwave will not penetrate the shell to do anything to the plates.
But it will render most of the control board unusable. The radiation can damage the controller chips, and arcing can damage the board tracks and passive components. But swapping the control board could make the data accessible again.
Actually, I've tested this. The platters inside are made of aluminium, glass, or a ceramic substrate. They are manufactured through a process called magnetron sputtering which coats the object with the magnetic material which is written to. This material arcs when photons of such frequencies (~2400 MHz) passes by. This results in actually damaging the disk and stripping the disk through "burning" or "arcing" the coated substrate. Thus, the disk would be classified unreadable but no longer usable.