Raspberry pi2 nas 2*2tb wd green

2TB via USB2...really?

I doubt those fans are necessary. But nice setup. Take a look at the XU4. It has 2 USB 3.0 ports which would take advantage of the speed.

What power supply is that???

Doesn't the drives need 12 volt rail?

I'm going to build a setup like yours, but with the Banana Pi with sata port instead.

You could have gotten a 12 volt power supply, and then use a step down (converter) to make it output both 12 volt and 5 volt

That way would also make you run your fans at 12 volt, if you wanted to.

Hope you get everything working as you want.