Walter, thank you for those links, the were very helpful.
It turns out that the location (remote from me) had a power outage, and the UPS strategy had not been implemented yet

. I'm guessing that there was some filesystem corruption on hda6.
This is what I did:
1. I ran fsck on /dev/hda6 (and fixed a whole bunch of errors)
2. I then added the partition back to the array with:
/sbin/raidhotadd /dev/md1 /dev/hda6
The disk is currently syncing now...
[root@zeus /sbin]# cat /proc/mdstat
Personalities : [raid1]
read_ahead 1024 sectors
md2 : active raid1 hdb1[1] hda1[0] 264960 blocks [2/2] [UU]
md0 : active raid1 hdb5[1] hda5[0] 15936 blocks [2/2] [UU]
md1 : active raid1 hda6[2] hdb6[1] 19727680 blocks [2/1] [_U] recovery=15% finish=142.8min
unused devices:
There is a good post at:
http://forums.contribs.org/index.php?topic=10221.msg38508#msg38508
that covers some of these items.
Cheers,
Andrew