Add a spare to a poorly defined raid1

Not knowing what I was doing, I created a RAID1 with a single drive and a spare, as below.

I'd like to add the spare to the array. How do I do this without rebooting?

Cheers,
Dave


# definitions of existing MD arrays
ARRAY /dev/md0 level=raid1 num-devices=1 UUID=9921faab:5d93510e:0db287f1:dc103b07
   spares=1

# mdadm --detail /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Fri Jul 18 14:57:40 2008
     Raid Level : raid1
     Array Size : 104320 (101.89 MiB 106.82 MB)
  Used Dev Size : 104320 (101.89 MiB 106.82 MB)
   Raid Devices : 1
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Wed Feb 15 03:37:11 2012
          State : clean
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

           UUID : 9921faab:5d93510e:0db287f1:dc103b07
         Events : 0.218

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1

       1       8       17        -      spare   /dev/sdb1