mdadm failed device

vvelichko:~# mdadm -Q --detail /dev/md1
/dev/md1:
        Version : 1.2
  Creation Time : Thu Oct 11 20:35:01 2012
     Raid Level : raid1
     Array Size : 488255296 (465.64 GiB 499.97 GB)
  Used Dev Size : 488255296 (465.64 GiB 499.97 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Fri Apr  3 19:23:00 2015
          State : active, degraded
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

           Name : vvelichko:1  (local to host vvelichko)
           UUID : 62682c99:dff5af61:58b9452d:08a77070
         Events : 20230880

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       0        0        1      removed

       2       8       33        -      spare   /dev/sdc1

В этих ваших интернетах написаны тонны непотребной херни, вплоть до ручной миграции данных с массива. Суть в строке "Total Devices : 2"

vvelichko:~# mdadm --grow --raid-devices=3 /dev/md1

ребилд запуститься автоматически:

vvelichko:~# mdadm -Q --detail /dev/md1
/dev/md1:
        Version : 1.2
  Creation Time : Thu Oct 11 20:35:01 2012
     Raid Level : raid1
     Array Size : 488255296 (465.64 GiB 499.97 GB)
  Used Dev Size : 488255296 (465.64 GiB 499.97 GB)
   Raid Devices : 3
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Fri Apr  3 19:29:26 2015
          State : clean, degraded, recovering
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

 Rebuild Status : 0% complete

           Name : vvelichko:1  (local to host vvelichko)
           UUID : 62682c99:dff5af61:58b9452d:08a77070
         Events : 20231187

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       2       8       33        1      spare rebuilding   /dev/sdc1
       2       0        0        2      removed