Depending on the quality of the hardware involved it could do little to no damage on a well protected system. Or do lots of damage, perhaps completely frying something cheap and poorly put together. But I’d expect I’d usually just fry the motherboard, leaving most other components intact for the most part.
In the case of spinning platter media, I’m guessing that it’d at worst fry the HDD’s motor. Which is inconvenient but by no means catastrophic.
ETA: I’m a helpdesk technician. So my knowledge is limited, but I’ve replaced one or two (literally, like two, not a bunch) of HDD motors. It’s not easy, and you have to observe the utmost clean handling rules, but besides that, if you can get the HDD apart cleanly and put in a new motor, the drive is likely fine, at least for a while.
