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]
More from Gizmodo
Does this only happen on Vista? On XP, my usb drives never seem to want to go to sleep. Strangely enough, my firewire connected drives sleep.