Want an efficient hard drive? Go with a 2.5 laptop drive. They consume less energy, and are designed to spin down and spin up many more times in their lifetimes than regular hard drives are.
Example:
I built a FreeNAS device with a VIA EPIA mini-itx motherboard and a laptop hard drive (SATA). With one case fan (the motherboard is passively cooled) and a compact flash chip to hold the FreeNAS OS, the system only uses 15 to 16 watts. NICE!!!
I may build another one of these using two laptop hard drives and create a RAID array.