So what failures are caused by humidity? Most of the electronics are sealed so I don’t expect that chips or capacitors dying would increase because of humidity. Is it cable connections? Inductors? Solder joints?
If I remember correctly, drives have vents to the outside world to regulate pressure.
Maybe condensation on the platters, which can happen when there is a change in temperature, causes errors and problems. The media is ferrous and maybe susceptible to moisture?
Only a guess.