{snippus maximus}
Out of interest, I wonder if
hdparm -I /dev/sda
shows anything interesting, adjusting to your drive names as appropriate.
I decided to google for "kernel: [49309.671222] Result: hostbyte=DID_OK driverbyte=DRIVER_TIMEOUT"
This thread was quite interesting http://ubuntuforums.org/archive/index.php/t-1681924.html Post by ashikaga at February 10th, 2011, 11:06 PM:
"Or, you're trying to use the Western Digital Green drives, and they're taking too long to respond and getting kicked out of the array. That's right - all the drives are WD20EARS, which I realise in hindsight was not a great choice for a RAID configuration!
I haven't yet had time to try replacing cables or switching SATA ports around; however, as suggested in another thread, I tried disabling NCQ on all three drives and re-created the array, and this morning it seemed to have re-built successfully! All three drives came up as 'active sync'. I might try failing the other two drives and re-adding them to make sure it still works, but assuming it does, is this likely to be a reliable fix? Any significant problems with disabling NCQ? (The server is never going to be under heavy load, just for home-use.)"
So, it seems that some WD Green drives have had problems, that seem to be solved (at least once) by disabling NCQ - Native Command Queuing.
NCQ - Native Command Queuing works like this: If you ask for sectors 2, 4, 3, 1, then without NCQ you get them in that order, but with NCQ, you get them in the order the drive thinks is most efficient - e.g. 1, 2, 3, 4.
I've seen a few posts like this one:
http://serverfault.com/questions/305890/poor-linux-software-raid-5-performan...
which says that NCQ is a bad thing in RAID 5 as it slows things down. I can see this could be an issue if both drives were asked for the info and returned it in a different order.
It appears that NCQ can be disabled by
$ echo 1 > /sys/block/sda/device/queue_depth $ echo 1 > /sys/block/sdb/device/queue_depth
(adjusting drive names appropriately)
Or more permanently as in this thread:
https://groups.google.com/forum/#!topic/linux.debian.user/kINSsYgJIes
It also occurs me that perhaps the drives have acoustic management (i.e. quiet mode) which may slow things down or cause problems. If it's still not working, perhaps try to turn that off too.
Good luck & keep us posted!
Steve