Bit flips like this helped me to discover that two of my 10 SATA cables were marginal. Since these are >2T drives, did you do anything with ashift? Depending on which ZFS implementation you're using, this question might not make sense? An array of drives where some are faking 512 byte sectors and ( some are really using 512 byte sectors OR some are using 4K sectors ) can cause abysmal performance. Thomas