According to Raid-failure.com the probability of a successful rebuild after 1 drive in a 5 drive array fails is 99.8%. "Highly probable" is way overstated.
dminches
The success probability of the rebuild process and the probability of more than one drive failing are two different things. One (1) disk fails in a RAID 5, true, it is 99.8% probable that the array can be rebuilt while never losing the use of your data. That is one of the thing RAID 5: it allows continuity of operation in the event of ONE (1) disk failure .. if 2 drives were to fail in RAID 5 (emphasis on 5) simultaneously or in an unfortunate sequence: Let's see the scenario:
You have ONE disk failure and promptly exchange the disk with a new in disk, your array is rebuilding.. you are happy. I is however a slow process. can take several days for a 12 TB array. You must pray that nothing happens to any one of the other disks during the rebuilding else you lose all your data. If two disks were to fail simultaneously in operation, you lose all data too.
In most cases people use the same drives to build a RAID 5 array increasing thus the probability of 2 drives going out at the same time. Interestingly SHR allows us to build a RAID-like array with dissimilar drives in some ways mitigating the risks of two drives going at the same time or in close sequence..
RAID 5 is protection and insurance for operation continuity RAID 5 doesn't replace Back-up... it is a serious difference.