The D5100 is a 5-bay DAS that supports RAID6. Whereas RAID5 - the usual low-cost method of ensuring reliability - uses a single parity block, RAID6 utilises two, meaning that an array can withstand two concurrent drive failures before the RAID cannot be rebuilt. The disadvantage lies with the amount of capacity that's required to be set aside, which can be considerable in a limited-drive scenario.
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