Hi,

The drive at /dev/sda3 has already been failed/removed. The reason was:

Buffer I/O error on device sda2, logical block 65536
lost page write due to I/O error on sda2
JBD2: Error -5 detected when updating journal superblock for sda2-8.

  What I would do is run smartctl -t short /dev/sdc, then smartctl -a /dev/sdc
and make sure the drive was reported as PASSED and not in imminent failure. Then
run fsck on /dev/sdc2 (the partition reporting the I/O error on what was sda2 --
DO NOT fsck /dev/sdc3.

Okay, ran a non-destructive badblocks after the smartctl test and it
passed. Rebooted and it switched back to sda from sdc. Ran "mdadm
--add /dev/md0 /dev/sda3" and it's now rebuilding.

I've done all this before, but somehow got a little nervous with it
being 3000 miles away and at a customer's location :-)

Thanks so much, everyone, for all your help.
Regards,
Alex