mdadm --detail /dev/md1
/dev/md1:
        Version : 1.2
  Creation Time : Fri Dec  7 18:11:05 2012
     Raid Level : raid1
     Array Size : 7807936 (7.45 GiB 8.00 GB)
  Used Dev Size : 7807936 (7.45 GiB 8.00 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Fri Jul 18 17:40:45 2014
          State : clean, degraded
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

           Name : debian:1
           UUID : 65edbf80:65c3568c:77fbba31:1184f6cb
         Events : 979

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

       2       8       19        -      spare   /dev/sdb3