quiet, low-power, always-available external storage

energy-efficient external hard drives:
Q: Is the power management a function of the drive itself, or of the enclosure, or of either?
Q: What is more significant: a low-power drive, or a well-managed drive? i.e., if I want quiet, low-power, always-available external storage, what am I really looking for?

A USB-powered drive would presumably be low-power, and if low-power, low-heat, so no fan, and also no power dissapation from the wall wart...

getusb.info says "bus-powered? fuggedaboudit."

Inside a desktop machine, a high-RPM drive can be (paradoxically?) more energy-efficient: it can spin up, hand off the data quickly, then go back to low-power sleep mode.