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: