As far as I know, the drive needs to support a low power state or a parked state when not being accessed. Windows will not turn off the drive. I only have one external drive, a WD MyBook, that spins down to a low power state when not in use and this is independent of any power management settings that I have configured. If your external disk is in a decent enclosure it has adequate ventilation. I've never had an external disk overheat.
-steve