Getting an energy efficient small server

For mirroring my backup drive, central data store for devices, music playing and a webserver for experiments, I'd like to run a small server at home. I want this server to be energy efficient, easy to modify, robust, silent and run customizable free software. It should have at least 500 GB of storage, but 1 or 1.5 TB is better. You can buy very low-energy computers such as the Fit-PC 2 (6 watt) or the Linutop 2 (8 watt). Energy costs for machines that run constantly can be roughly estimated by doubling the power draw in watt, so running a device that uses 8 watt constantly costs about 16 euro a year.

Until recently the computer I used most was a Dell X1 Latitude laptop. That machine is now 4.5 years old. At the time, I chose it because it is a laptop with no fan and hence very silent. It is still better than any atom based netbook. So I would like to use this laptop as a server. UPS and screen are integrated which is a nice plus. The machine has a 1.8" disk built in. It is not possible to replace it with a disk of at least 500 GB. I wanted to know the energy cost of adding more storage to the X1. So I did some power measurements with an 2.5" external disk (Toshiba, 160 GB) and a 3.5" external disk (TrekStore 500GB). I measured on my current main laptop, a Lenovo X200s too.

Lenovo X220s (console, idle, low brightness unless otherwise specified) Adapter only: 6 W Console, low brightness: 19 W Console, high brightness: 21 W 100% cpu and high brightness: 40 W mounted 2.5" disk: 24 W active (dd) 2.5" disk: 28 W mounted 3.5" disk: 37 W active (dd) 3.5" disk: 41W

Dell Latitude X1 (console, idle, low brightness unless otherwise specified) Adapter only: 0 W Console, low brightness: 15 W Console, high brightness: 19 W 100% cpu and high brightness: 23 W mounted 2.5" disk: 17 W active (dd) 2.5" disk: 21 W mounted 3.5" disk: 32 W active (dd) 3.5" disk: 37 W

The 2.5" disk uses USB for power. The 3.5" disk has a separate adapter which is included in the power measurements. The device used for the power measurements is a DEM1379. The idle 3.5" drive uses 13-15 more watt and the active drive uses 13-16 more watt. The difference is as large as power usage of the entire server. So I am now wondering if there are more energy efficient external 3.5" drives.