Comment 1 for bug 1164008

I'm digging into this some more and it seems that mdadm in Ubuntu 12.04 is detecting an old unused superblock on /dev/sdb that mdadm in Ubuntu 10.04 ignored.

$ mdadm --examine --scan --verbose
ARRAY /dev/md1 level=raid1 num-devices=2 UUID=5fb6928e:89f09cde:d0aebb6f:e3c24c31
   spares=1 devices=/dev/sdb
ARRAY /dev/md0 level=raid1 num-devices=2 UUID=a4176cee:096f0ba6:40663db4:2b8b6d2c
   devices=/dev/sdc1,/dev/sdb1

When we examine /dev/sdb, we see there's an old superblock there (note the update date is Thu Jul 7 09:00:21 2011).

$ mdadm --examine /dev/sdb
/dev/sdb:
          Magic : a92b4efc
        Version : 0.90.00
           UUID : 5fb6928e:89f09cde:d0aebb6f:e3c24c31
  Creation Time : Fri Jul 1 10:00:58 2011
     Raid Level : raid1
  Used Dev Size : 1953513408 (1863.02 GiB 2000.40 GB)
     Array Size : 1953513408 (1863.02 GiB 2000.40 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 1

    Update Time : Thu Jul 7 09:00:21 2011
          State : clean
 Active Devices : 1
Working Devices : 2
 Failed Devices : 1
  Spare Devices : 1
       Checksum : 57d747d3 - correct
         Events : 24

      Number Major Minor RaidDevice State
this 2 8 16 2 spare /dev/sdb

   0 0 8 33 0 active sync /dev/sdc1
   1 1 0 0 1 faulty removed
   2 2 8 16 2 spare /dev/sdb