Suppose we have eight data disks numbered 1 through 8, and three redundant disks: 9. 10, and 11. Disk 9 is a parity check on disks 1 through 4, and disk 10 is a parity check on disks 5 through 8. If all pairs of disks are equally likely to fail simultaneously, and we want to maximize the probability that we can recover from the simultaneous failure of two disks, then on which disks should disk 11 be a parity check?

Thanks.