> If they were powered down, it would take too long to retrieve data. It would be like Amazon Glacier where it can take 15 minutes to get a single image back.

Well, speaking for myself, I don't mind if I have to restore a backup, and have to wait 1 minute extra for a system to come online (that's about the typical boot time of a Linux system, so I suppose for a storage pod it would be about the same).

By the way, it seems possible to power down an individual SATA drive if it is not in use: [1].

[1] https://unix.stackexchange.com/questions/112117/shutdown-my-...