"Hibernate" and "standby" are different beasts. "Hibernate" essentially shuts down the computer but stores everything from ram to disk to allow you to restore your working state. "Standby" powers down most drives and the monitor but the machine stays on, so you get a much faster wake time. Standby might still be noisier than you'd like if your pc fans are noisy though.
Normally (in a desktop) the HD is powered from the main power supply, so shutting the system down will shut it down as well. With an external HD you get an external power adapter, so the situation is not quite the same. I will try to experiment with an external drive at work and let you know what I find. Since the drive itself is the same I would imagine you can still somehow signal it to spin down.
