As others said, spin down the drives when they're not in use. Make sure power saving is enabled on the drives and tune them to spin down after some appropriate amount of time. (hdparm lets you customize it on Linux)
Consider also sleeping the NAS when not in use. You can try using Wake-on-LAN to remotely wake it up when you need to use it. Saves on electricity and heat! You could also sleep it on a schedule, in case you need to be online for backups to run at particular times.
