Yeah, I was thinking that. However a quick google says that the average laptop drive uses around 4W when accessing and 1.5W when idle. I'm also measuring at the mains point, and I'm not sure how efficient the transformer is - they're usually only about 75% efficient.

I'm loathe to have the drives regularly spin down. That sounds like a good way to make them wear out quicker. Though spinning up twice a day might not be too bad.