>On consumer-grade SATA drives that have a URE rate of 1 in 10^14, that means if the data on the surviving drives totals 12TB, the probability of the array failing rebuild is close to 100%.

10^14 bits is 12.5 TB, so on average, the chance of 12TB being read without a single URE is very low, and the probability the array fails to rebuild is close to 100%. I was estimating 10^14 bits to be about 12TB, so the probability is actually 12/12.5 = 96% chance of failure.

>...he has 12x 4TB drives. Once two drives failed, assuming he is using enterprise drives...there is a 33% chance the entire array fails if he tries to rebuild. If the drives are plain SATA, there is almost no chance the array completes a rebuild.

A RAID6 with two failed drives is effectively the same situation as a RAID5 with one failed drive. In order to rebuild one failed drive, the RAID controller must read all data from every surviving drive to recreate the failed drive. In this case, there are 10x 4TB surviving drives, meaning 40TB of data must be read to rebuild. Because these drives are presumably enterprise quality, I am assuming they are rated to fail reading one sector for every 10^15 bits read (10^15 bits = 125 TB). So it's actually 40/125 = 32% chance of failure if you try to rebuild.