nas ~ # mdadm --verbose --manage --set-faulty /dev/md2 /dev/sdd2
mdadm: set /dev/sdd2 faulty in /dev/md2
nas ~ # mdadm -D /dev/md2
/dev/md2:
        Version : 1.2
  Creation Time : Tue Aug 28 04:45:52 2012
     Raid Level : raid5
     Array Size : 78646272 (75.00 GiB 80.53 GB)
  Used Dev Size : 26215424 (25.00 GiB 26.84 GB)
   Raid Devices : 4
  Total Devices : 6
    Persistence : Superblock is persistent

    Update Time : Tue Dec 10 19:46:24 2013
          State : clean, degraded, resyncing (DELAYED)
 Active Devices : 3
Working Devices : 5
 Failed Devices : 1
  Spare Devices : 2

         Layout : left-symmetric
     Chunk Size : 512K

           Name : localhost.localdomain:2
           UUID : a9d908f3:f8632fcf:59a69085:83d5f6aa
         Events : 1045

    Number   Major   Minor   RaidDevice State
       0       8        3        0      active sync   /dev/sda3
       6       8       83        1      spare rebuilding   /dev/sdf3
       2       8       34        2      active sync   /dev/sdc2
       4       8       18        3      active sync   /dev/sdb2

       1       8       50        -      faulty   /dev/sdd2
       5       8       67        -      spare   /dev/sde3