cat /proc/mdstat
Personalities : [raid1]

md2 : active raid1 sda3[0] sdb3[2](S)
      1458319504 blocks super 1.0 [2/1] [U_]

md1 : active raid1 sda2[3] sdb2[2]
      524276 blocks super 1.0 [2/2] [UU]

md0 : active raid1 sda1[0] sdb1[2]
      6291444 blocks super 1.0 [2/2] [UU]

mdadm --detail /dev/md2
/dev/md2:
        Version : 1.0
  Creation Time : Fri May 24 11:22:21 2013
     Raid Level : raid1
     Array Size : 1458319504 (1390.76 GiB 1493.32 GB)
  Used Dev Size : 1458319504 (1390.76 GiB 1493.32 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon Aug  4 22:08:23 2014
          State : clean, degraded
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

           Name : rescue:2  (local to host rescue)
           UUID : 96b46a6c:f520938c:f94879df:27851e8a
         Events : 616

    Number   Major   Minor   RaidDevice State
       0       8        3        0      active sync   /dev/sda3
       1       0        0        1      removed

       2       8       19        -      spare   /dev/sdb3

mdadm --manage /dev/md2 --add /dev/sdb3

mdadm --manage /dev/md2 --remove /dev/sdb3

mdadm --stop /dev/md2 ; mdadm --start /dev/md2

mdadm --assemble --run --force --update=resync /dev/md2 /dev/sda3 /dev/sdb3