nas ~ # mdadm -D /dev/md0
/dev/md0:
        Version : 1.0
  Creation Time : Fri Aug 24 04:18:31 2012
     Raid Level : raid1
     Array Size : 511988 (500.07 MiB 524.28 MB)
  Used Dev Size : 511988 (500.07 MiB 524.28 MB)
   Raid Devices : 2
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Tue Dec 10 19:35:24 2013
          State : clean
 Active Devices : 2
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 2

           Name : localhost.localdomain:0
           UUID : 262c8e68:c7c4fa6d:19af27e8:0cfc68a1
         Events : 178

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       2       8       49        1      active sync   /dev/sdd1

       3       8       33        -      spare   /dev/sdc1
       4       8       17        -      spare   /dev/sdb1
nas ~ # mdadm --verbose --manage /dev/md0 --remove /dev/sdb1 /dev/sdc1
mdadm: hot removed /dev/sdb1 from /dev/md0
mdadm: hot removed /dev/sdc1 from /dev/md0
nas ~ # mdadm --verbose --manage /dev/md0 --add /dev/sdf1 /dev/sde1
mdadm: added /dev/sdf1
mdadm: added /dev/sde1
nas ~ # mdadm -D /dev/md0
/dev/md0:
        Version : 1.0
  Creation Time : Fri Aug 24 04:18:31 2012
     Raid Level : raid1
     Array Size : 511988 (500.07 MiB 524.28 MB)
  Used Dev Size : 511988 (500.07 MiB 524.28 MB)
   Raid Devices : 2
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Wed Dec 11 19:53:51 2013
          State : clean
 Active Devices : 2
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 2

           Name : localhost.localdomain:0
           UUID : 262c8e68:c7c4fa6d:19af27e8:0cfc68a1
         Events : 182

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       2       8       49        1      active sync   /dev/sdd1

       3       8       81        -      spare   /dev/sdf1
       4       8       65        -      spare   /dev/sde1
nas ~ # mdadm --verbose --manage --set-faulty /dev/md0 /dev/sdd1
mdadm: set /dev/sdd1 faulty in /dev/md0
nas ~ # mdadm -D /dev/md0
/dev/md0:
        Version : 1.0
  Creation Time : Fri Aug 24 04:18:31 2012
     Raid Level : raid1
     Array Size : 511988 (500.07 MiB 524.28 MB)
  Used Dev Size : 511988 (500.07 MiB 524.28 MB)
   Raid Devices : 2
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Wed Dec 11 19:55:33 2013
          State : clean
 Active Devices : 2
Working Devices : 3
 Failed Devices : 1
  Spare Devices : 1

           Name : localhost.localdomain:0
           UUID : 262c8e68:c7c4fa6d:19af27e8:0cfc68a1
         Events : 201

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       4       8       65        1      active sync   /dev/sde1

       2       8       49        -      faulty   /dev/sdd1
       3       8       81        -      spare   /dev/sdf1
nas ~ # mdadm --verbose --manage /dev/md0 --remove /dev/sdd1
mdadm: hot removed /dev/sdd1 from /dev/md0