In a RAID 10 array of four disks (two mirrored pairs, striped across the two pairs), two of the four disks fail at the same time. If every pair of disks is equally likely to be the one that fails, which is the probability that data is lost?

Answer: B. 1/3

There are C(4, 2) = 6 ways to choose the two failing disks out of four. Data is lost when both disks of the same mirrored pair fail, and since there are two mirrored pairs, that happens in 2 ways. The probability of data loss is therefore 2 ÷ 6 = 1/3. In the remaining 4 ways, one disk remains in each mirrored pair and no data is lost, so RAID 10 can survive two failures depending on the combination, unlike RAID 6, which survives any two.

• A. This counts only one mirrored pair. There are two fatal combinations.
• B. Correct. Of the 6 ways, data is lost in the 2 where both disks of the same mirrored pair fail.
• C. This is the probability that no data is lost (4 ÷ 6).