I had created two 2TB HDD partitions (/dev/sdb1 and /dev/sdc1) in a RAID 1 array called /dev/md0 using mdadm on Ubuntu 12.04 LTS Precise Pangolin.

The command sudo mdadm --detail /dev/md0 used to indicate both drives as active sync.

Then, for testing, I failed /dev/sdb1, removed it, then added it again with the command sudo mdadm /dev/md0 --add /dev/sdb1

watch cat /proc/mdstat showed a progress bar of the array rebuilding, but I wouldn't spend hours watching it, so I assumed that the software knew what it was doing.

After the progress bar was no longer showing, cat /proc/mdstat displays:

md0 : active raid1 sdb1[2](S) sdc1[1]
      1953511288 blocks super 1.2 [2/1] [U_]

And sudo mdadm --detail /dev/md0 shows:

/dev/md0:
        Version : 1.2
  Creation Time : Sun May 27 11:26:05 2012
     Raid Level : raid1
     Array Size : 1953511288 (1863.01 GiB 2000.40 GB)
  Used Dev Size : 1953511288 (1863.01 GiB 2000.40 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon May 28 11:16:49 2012
          State : clean, degraded 
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

           Name : Deltique:0  (local to host Deltique)
           UUID : 49733c26:dd5f67b5:13741fb7:c568bd04
         Events : 32365

    Number   Major   Minor   RaidDevice State
       1       8       33        0      active sync   /dev/sdc1
       1       0        0        1      removed

       2       8       17        -      spare   /dev/sdb1

I've been told that mdadm automatically replaces removed drives with spares, but /dev/sdb1 isn't being moved into the expected position, RaidDevice 1.

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UPDATE (30 May 2012): A badblocks destructive read-write test of the entire /dev/sdb yielded no errors as expected; both HDDs are new.

As of the latest edit, I assembled the array with this command:

sudo mdadm --assemble --force --no-degraded /dev/md0 /dev/sdb1 /dev/sdc1

The output was:

mdadm: /dev/md0 has been started with 1 drive (out of 2) and 1 rebuilding.

Rebuilding looks like it's progressing normally:

md0 : active raid1 sdc1[1] sdb1[2]
      1953511288 blocks super 1.2 [2/1] [U_]
      [>....................]  recovery =  0.6% (13261504/1953511288) finish=2299.7min speed=14060K/sec

unused devices: <none>

I'm now waiting on this rebuild, but I'm expecting /dev/sdb1 to become a spare just like the five or six times that I've tried rebuilding before.

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UPDATE (31 May 2012): Yeah, it's still a spare. Ugh!

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UPDATE (01 June 2012): I'm trying Adrian Kelly's suggested command:

sudo mdadm --assemble --update=resync /dev/md0 /dev/sdb1 /dev/sdc1

Waiting on the rebuild now...

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My questions are:

1. Why isn't the spare drive becoming active sync?
2. How can I make the spare drive become active?