If we assume for the sake of argument simple RAID 6 setup, to lose say a block of data you'd need to * have a bad block on one drive * have a bad block in *EXA

If we assume for the sake of argument simple RAID 6 setup, to lose say a block of data you'd need to * have a bad block on one drive * have a bad block in *EXACTLY SAME LOCATION* on the second drive * have a bad block in *EXACTLY SAME LOCATION* on the third drive * all of that happening within one scrub period (which is usually once a month). And that's just simple RAID6 setup without external redundancy. As you can see it would be far easier to lose data due to just correlated drive failures, than from random block corruption.