2b- LARGE UPS because HDs are the components that have the higher power consomption (a 700VA UPS gives me about 10-12 minutes on a machine with a XP2200+, 1GB RAM and a 40GB HD, however this fall to...... less than 25 secondes with seven HDs ! all ATA),
I got my hands on a (free) 1400 VA APC rackmount UPS ; the batteries were dead so I stuck two car batteries in. It can power my computer (Athlon 64, 7 drives) for more than 2 hours... It looks ugly though. I wouldn't put this in a server rack, but for my home PC it's perfect. It has saved my work many times...
Harddisks suck in about 15 watts each, but draw large current spikes on seeking, so the VA rating of the UPS is important. I guess in your case, the batteries have enough charge left; but the current capability of the UPS is exceeded.
Some hardware ctrlrs are able to avoid the loss of a disk if you turn to have some faulty sectors (by relocating internally them); software RAID doesn't as sectors *must* be @ the same (linear) addresses.
Harddisks do transparent remapping now... linux soft raid can rewrite bad sectors with good data and the disk will remap the faulty sector to a good one.