Build an identical one and keep it far enough away that you need to feel safe? Ideally at least a few blocks away, sync them over a short-haul wireless link. (encrypted of course!) and take the same precautions as you would with anything else?
Oh yeah don't do a flat fire store, make it a SVN repository of course.
I did someting some years ago with 200GB (and later 500GB) drives:
10 drives in a chieftec Big tower. 6 drives go into the two internal drive cases, 4 go into a 4-for-3 mounting with a 120mm fan. Controller: 2 SATA on board and 2 x Promise 4 port SATA conroller 300 TX4 (a lot cheaper than Arcea and kernel native support). Put Linux software RAID 6 on the drives, spare 1 GB or so per drive for RAID1 (n-way) system. Done.
I say you've got it close, except use ZFS instead. Solaris on x86 isn't that bad, especially if you don't plan on doing anything else with the hardware beyond file-serving duties!
10 to the 6th power Bicycles = 2 megacycles