I've built a couple NAS machines off Atom Mini-ITX boards, and in one case I supplied the mainboard and the two disks using a PicoPSU 12V adapter. Once the syst

I've built a couple NAS machines off Atom Mini-ITX boards, and in one case I supplied the mainboard and the two disks using a PicoPSU 12V adapter. Once the system accepts 12V, the UPS can be a car/motorcycle battery left inline as buffer, so that in case of blackout there's no switching involved. Not compact as the Helios64 UPS, but it works and is less risky than having two Lithium cells near the drives. I didn't try this configuration as my PicoPSU needed quite accurate 12V input, which the car batteries would exceed when fully charged, but their M3-ATX Automotive model accepts from 6 to 24 volts making ideal, as the name implies, for being supplied by a car battery. Regarding the 5 bay storage, you can buy backplanes with pull out bays for 5*3.5 disks that would fit into 3*5.25 bays. A search for "3 to 5 sata backplane" returns some products.