Why do I keep talking about laptop drives:
A laptop drive in a fully active Read/Write state uses only 3 watts and can idle at less than 1Watt and when it spins down is next to nothing at all, and has ten times the life span of a desktop drive in terms of spin up/down toggles, but they are a lot more expensive per GB than a standard 3.5" desktop drive.
So extending the life of your server drives, getting better value for money and saving power can be reached by combining the 2 types of drive.
The bulk of your storage should sit on a 3.5" desktop drive, cheap storage. The server should spin down this power hungry drive after it has not been used for some thing like 1 hour, this ensure it is not spinning up/down all day, but saves loads of power when inactive for periods of time (You go to work and sleep at night, don't you?).
The Operating System should live on a 2.5" laptop drive which will be on for most of the time (because of writing to log files) but could handle a really short spin down inactivity time if you wished. This greatly reduced the idle power of your PC, assuming that you used a low power mother board and cpu.
Power Dissipation of laptop drive from Western Digital
Power Dissipation of desktop drive from Western Digital