Lifehacker found this NoSleepHD utility, which stops external (USB) hard drives from powering down to save energy.

This saves precious seconds if you're only accessing the drive every few minutes, causing it to shut down between each use. It also helps avoid problems if your media server has an issue with drives going to sleep and not coming back on, but may shorten the overall lifespan of your drives. [NoSleepHD via GHacks via Pashhacker]