I am not sure if I should run a command to power down the disk before removing it.
But, for whatever reason, removing it and putting it back caused a logical drive to fail. Annoying because there was nothing actually wrong, and that can only be fixed with a restart and to accept "data loss" (none). FYI it's a HPE Smart Array (Smart Array P420i Controller).
I'm not sure I want to try hot swapping any time soon. :/
I had to migrate lots of data to another box for a few minutes to restart. Argh.