Ummm, regarding Vendetta'soriginalanswer: Your total of 12 possible drives to be lost neglected to take into account this following case...

If you lost 2 drives in each of the individual RAID 6 arrays, that gives you 8. Now, you could technically lose 4 more drives as you said before, however, each pair of these would have to be located together within exactly 2 of the RAID 6 arrays. If, for example, you lost one more drive in each of the RAID 6 arrays for a total of 3 drives per RAID 6 array, your RAID 6+6 array would lose all of it's RAID 6 arrays and cease to function. Similarly, if you lost 4 drives from one RAID 6 array, 2 from another, and 3 from each of the remaining 2 RAID 6 arrays, then 3 of the RAID 6 arrays would fail causing the RAID 6+6 array to lose its Parity drives and one of it's main drives cauing it to fail again.

Therefore, if you're still following, the maximum number of drivesthat can be lost SAFELY would be 10: 2 from each RAID 6, and 2 more either both in one RAID 6 array (losing one part of the RAID 6+6) or 1 in each of 2 of the RAID 6 arrays (losing 2 RAID 6 arrays from the RAID 6+6 but still leaving 2 for functionality).

So, to make a long story short, if you lose more than 10 arrays, you may possibly compromise your entire array but with 10 or less you will never lose your RAID 6+6 array.

Good luck deciphering that one!