Simultaneous disk failure

Modern hard drives have annual failure rate of about 5%. With this sort of the failure probability you cannot realistically talk about the simultaneous hard drive failures, even if the window of simultaneousness spans several hours. The simultaneous failure of two drives is just not probable enough. The catch is, the statistics only applies to independent drives. The common-mode failure, when several drives fail because of a single cause, is not accounted for.

As a consequence of this, in fault-tolerant RAID applications more effort should be put to eliminate possible single points of failure. Realistically, this means the most effective thing you can do is to eliminate the operator, eh.