I think the concern is that if you have two disks die it is not a big deal, unless those two disk happen to be of the same pair[1], then most likely all data is

I think the concern is that if you have two disks die it is not a big deal, unless those two disk happen to be of the same pair[1], then most likely all data is lost. [1] if you're not careful, this is actually very likely to happen. If you purchase 2 disks at the same time, and they are under the same usage patterns (which they will be when working in a pair), under the same temperature then they are very likely to die at the same time.