Ars Technica walks through building a Windows NAS:
At this point, I had a couple of options. I could spend more money on a better, faster NAS, one that wouldn’t disappoint me with its performance. Or… I could go ahead and build my own, which would give me the flexibility to build basically whatever box I wanted. Inspired by our recent articles on building a living room gaming PC and a DIY router, I decided to take the more Ars option.
Source: The ins and outs of planning and building your own home NAS | Ars Technica
TechSpot combines some popular NAS components, a Silverstone DS380 case, Asrock C2750D4I motherboard, and FreeNAS.
Assembling your own NAS would net more performance as well because you’d be using a Celeron or Pentium over the Atom or other SoCs, while power shouldn’t be a concern with Haswell using less than 30 watts at idle. As the cherry on top, open source software such as FreeNAS and enclosures like Silverstone’s DS380 should make it less daunting to get started with your homebrewed eight-bay NAS server.
bit-tech.net has a new article on building a FreeNAS box, including choosing hardware and installing the software.