Seagate drives often cause:

[   23.064454] ata11: exception Emask 0x10 SAct 0x0 SErr 0x10000 action 0xe frozen
[   23.064459] ata11: SError: { PHYRdyChg }
[   23.064468] ata11: hard resetting link

So far there's no good fix other than replacing the drive...

see
http://www.anginf.de/index.php?project/sataraid5
http://www.gossamer-threads.com/lists/linux/kernel/990682
http://xfs.org/index.php/XFS_FAQ#Q:_How_can_I_address_the_problem_with_the_disk_write_cache.3F