I was thinking that the specs were a bit stingy compared to the QNAP I have sitting here (2GHz ARM, 512MB of RAM).
Looks awful unless you're into white plastic bricks but has all the functionality including Time Machine and NFS. I liked it so much I bought another one to back it up.
Now have my stuff on at least five hard drives so fairly secure unless there's a fire or Johnny Darko incident...
Start by stripping down two old desktop Celeron (or Via C3) PCs and bundle their motherboards into a 3U rack case (stick one mobo to the bottom the normal way up and then the other to the inside of the case top upside-down), put in two low-power desktop PSUs, some fans, some cheap networking cards (most desktop PCs have at least three PCI slots, you will need two LAN cards per mobo for redundant LAN links for the cluster) and a SCSI card per mobo (old Adaptec cards are always on e Bay and usually work fine with Linux).
Then add some SCSI disks (best four per mobo, total eight), split into two chains off the SCSI cards (you may decide to buy a pair of old SCSI shelves like the hp ds2100, again cheap on e Bay).
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