Spare disk not becoming active

[Tudor Holton <tudor@smartguide.com.au>]

Hallo,

I'm having some trouble with an array I have that has become degraded.

I have an array with this array state:

md101 : active raid1 sdf1[0] sdb1[2](S)
       1953511936 blocks [2/1] [U_]


mdadm --detail says:

/dev/md101:
         Version : 0.90
   Creation Time : Thu Jan 13 14:34:27 2011
      Raid Level : raid1
      Array Size : 1953511936 (1863.01 GiB 2000.40 GB)
   Used Dev Size : 1953511936 (1863.01 GiB 2000.40 GB)
    Raid Devices : 2
   Total Devices : 2
Preferred Minor : 101
     Persistence : Superblock is persistent

     Update Time : Fri Nov 23 03:23:04 2012
           State : clean, degraded
  Active Devices : 1
Working Devices : 2
  Failed Devices : 0
   Spare Devices : 1

            UUID : 43e92a79:90295495:0a76e71e:56c99031 (local to host 
barney)
          Events : 0.2127

     Number   Major   Minor   RaidDevice State
        0       8       81        0      active sync /dev/sdf1
        1       0        0        1      removed

        2       8       17        -      spare   /dev/sdb1


If I attempt to force the spare to become active it begins to recover:
$ sudo mdadm -S /dev/md101
mdadm: stopped /dev/md101
$ sudo mdadm --assemble --force --no-degraded /dev/md101 /dev/sdf1 /dev/sdb1
mdadm: /dev/md101 has been started with 1 drive (out of 2) and 1 spare.
$ cat /proc/mdstat
md101 : active raid1 sdf1[0] sdb1[2]
       1953511936 blocks [2/1] [U_]
       [>....................]  recovery =  0.0% (541440/1953511936) 
finish=420.8min speed=77348K/sec

This runs for the allotted time but returns to the state of spare.

Neither disk partition report errors:
$ cat /sys/block/md101/md/dev-sdf1/errors
0
$ cat /sys/block/md101/md/dev-sdb1/errors
0

Are there mdadm logs to find out why this is not recovering properly?  
How otherwise do I debug this?

Cheers,
Tudor.