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 : Mon Nov 25 21:35:39 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