Fault Tolerant RAID: Cold Spare Advised
When using fault tolerant RAID, the smart move is to have a cold spare on hand.
For example, a RAID-5 reverts to RAID-0 when a drive fails. You want that puppy rebuilt ASAP, to restore fault tolerance (failure of a 2nd drive would mean loss of the RAID).
In plain language, this means that when purchasing a unit like the 4-drive OWC Thunderbay to be used as a RAID-5 or RAID-1+0, buy a 5th spare drive and pre-test and pre-initialize it.