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Probability, or event?
I don't agree. For a given quantity of drives there will be a probability that two of them will fail at the same time, where "the same time" is defined as the period of time it takes to rebuild a single drive in a RAID 5. Simply dividing the drives up into subsets does not change that probability. And, without prior knowledge, there is no way to know which of the drives will fail, so there is no way to divide the failures among the subsets. A double drive failure in a RAID 5 will kill the array, and if that RAID 5 is in a RAID 50 it will kill the RAID 50. There is no change in the level of risk.