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I recently replaced a drive (/dev/sdb). All arrays have been rebuilt correctly except for one of them (/dev/md2). There is no problem with the new drive. Partitions are OK. mdadm says it's "spare rebuilding" but apparently it isn't happening.

# mdadm --detail /dev/md2
/dev/md2:
        Version : 1.2
  Creation Time : Mon Oct  8 21:47:44 2012
     Raid Level : raid1
     Array Size : 1048574840 (1000.00 GiB 1073.74 GB)
  Used Dev Size : 1048574840 (1000.00 GiB 1073.74 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Wed Nov 19 00:48:20 2014
          State : clean, degraded
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

           Name : rescue:2
           UUID : 3c3c297e:99969855:e5fee8ab:457adcef
         Events : 301

    Number   Major   Minor   RaidDevice State
       0       8        3        0      active sync   /dev/sda3
       2       8       19        1      spare rebuilding   /dev/sdb3

# cat /proc/mdstat 
Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md3 : active raid1 sda4[0] sdb4[2]
      832587071 blocks super 1.2 [2/2] [UU]

md2 : active (auto-read-only) raid1 sda3[0] sdb3[2]
      1048574840 blocks super 1.2 [2/1] [U_]

md1 : active raid1 sda2[0] sdb2[2]
      1048574840 blocks super 1.2 [2/2] [UU]

md0 : active raid1 sda1[2] sdb1[3]
      524276 blocks super 1.2 [2/2] [UU]

unused devices: 

3

It's probably not taking any action to rebuild because it's in auto-read-only state.

Verify that there's no nasty reason it's gone into that state, verify that your backups are good before you do anything, and then set it to writable with mdadm --readwrite /dev/md2

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