I personally know that the following hard drives have the internal firmware capabilty to spin down (aka. "go into sleep mode"), which saves energy:

Seagate External Hard Drives

Maxtor OneTouch II

This capability may not work without a little hassle: apparently the drive will be kept awake by periodic activity from the NSLU2 unless modifications are made. This article discusses the subject relative to the Maxtor drive. Perhaps some knowlegeable soul will send me instructions on how to incorporate this feature into this wiki's recipe.

One TwonkyVision forum participant wrote recently that in his experience using the RAM installation allows spin-down to work. Time will tell.