Comment 31 for bug 316670

During boot I was prompted with a warning, no valid partitions found on /dev/sdc, hit ctrl-D to continue. Because this isn't a root partition, I was able to continue booting with ctrl-D, which took me all the way to the gdm login. I then dropped to a shell with ctrl-alt-F1. There were no questions about degraded mode booting.

I fixed my problem with the following steps:

1. Stop the false md_d0 array
$ sudo mdadm --stop /dev/md_d0

2. Assemble the degraded md0 (only sdb was found) and mount it
$ sudo mdadm --assemble --scan
$ sudo mount /dev/lvm-raid/homelv /home

3. use mkconf to find the configuration option
$ sudo /usr/share/mdadm/mkconf

4. gedit /etc/mdadm/mdadm.conf to include the output from mkconf AND to use only sdb
DEVICE /dev/sdb*
# definitions of existing MD arrays
ARRAY /dev/md0 level=raid1 num-devices=2 UUID=6a291d6f:27049e9f:b95e1135:52246c3f

5. Reboot --> ubuntu found and mounted my array just fine (mount info for LVM is my fstab)

6. Re-add sdc to md0
$ sudo mdadm /dev/md0 --re-add /dev/sdc1

7. Watch rebuild!
$ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 sdb1[0] sdc1[2]
      976759936 blocks [2/1] [U_]
      [==>..................] recovery = 14.2% (139109312/976759936) finish=381.7min speed=36570K/sec

unused devices: <none>

I'm not sure why mdadm wanted to build the (incorrect) md_d0 and not the (correct) md0.