I recently had a RAID1 volume fail.  It had three primary partitions in it, all participating in separate, but similarly configured VGs.  This configuration comes from the fact that the machine is easily a decade old, having upgraded from multiple single disks to multiple VGs with arrays.I replaced the disk, created the partitions and added them back to the RAID arrays using mdadm.  This worked for md0 and md1, but for md2, it's failed twice.  This is the largest volume, it fails without error and is left in this state:# mdadm --examine /dev/sdc3/dev/sdc3:          Magic : a92b4efc        Version : 0.90.00           UUID : <omitted>  Creation Time : Wed Nov  3 10:56:13 2010     Raid Level : raid1  Used Dev Size : 1914417728 (1825.73 GiB 1960.36 GB)     Array Size : 1914417728 (1825.73 GiB 1960.36 GB)   Raid Devices : 2  Total Devices : 2Preferred Minor : 2    Update Time : Tue Oct 21 12:25:59 2014          State : clean Active Devices : 1Working Devices : 2 Failed Devices : 1  Spare Devices : 1       Checksum : 5d247e27 - correct         Events : 188268      Number   Major   Minor   RaidDevice Statethis     2       8       35        2      spare   /dev/sdc3   0     0       8        3        0      active sync   /dev/sda3   1     1       0        0        1      faulty removed   2     2       8       35        2      spare   /dev/sdc3How can I have a spare disk if the RAID 1 array has only one working disk?There are some real gotchas on this one though:The old disk has 512byte physical sectors, the new one 4096byte physicalThe old disk has a conventional MBR partition, the new one a GPT partitionThe old disk is 2TB, the new one 3TB.The plan for the 3TB is to eventually replace the other 2TB disk and expand the array, but for now, it's unused space.Note:Old configuration:  RAID1, 1x2TB disk, 512 byte physical sectors, MBR from 2010                           1x2TB disk, 512 byte physical sectors, MBR from 2010 (failed)New configuration:  RAID1, 1x2TB disk, 512 byte physical sectors, MBR from 2010                           1x3TB disk, 4096 byte physical sectors, GPT brand newThe 2TB disks were purchased in 2010, they've had a long run.  One failed, so I replaced it with a 3TB disk.   The GPT and physical partition thing might be a red herring.  Just mentioning it in case it strikes anyone as important.Some messages are appearing in syslog,Oct 21 18:02:58 blacktower kernel: [169469.308398] mdadm: sending ioctl 800c0910 to a partition!Oct 21 18:02:58 blacktower kernel: [169469.308402] mdadm: sending ioctl 800c0910 to a partition!Oct 21 18:03:54 blacktower kernel: [169525.933975] mdadm: sending ioctl 1261 to a partition!Oct 21 18:03:54 blacktower kernel: [169525.933978] mdadm: sending ioctl 1261 to a partition!Oct 21 18:03:54 blacktower kernel: [169525.934250] mdadm: sending ioctl 1261 to a partition!Oct 21 18:03:54 blacktower kernel: [169525.934253] mdadm: sending ioctl 1261 to a partition!This thread suggests that it's harmless, http://www.spinics.net/lists/raid/msg44887.htmlMay not be related, but it's possible that there's an issue in the kernel verison.  I'm on Squeeze LTS.  # uname -r2.6.32-5-686Hmm.. 
Spare disk in a 2-disk mdadm RAID1 array?
mdadm;disk;gpt;raid1