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I have two drives that used to be part of a single RAID1 volume but it appears that one of them went offline for some time, something I've noticed just now when I rebooted my system.

I now seem to have two RAID volumes, as reported by:

# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md126 : active raid1 sdc[1]
      2096116 blocks super 1.2 [2/1] [_U]

md127 : active (auto-read-only) raid1 sdb[0]
      2096116 blocks super 1.2 [2/1] [U_]

unused devices: <none>

Not exactly sure where to go from here. How can I merge and re-sync these volumes without data loss?


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First of all you have to find out which of the two devices has the more recent copy of the RAID. Try to mount them read-only and look at the files. The logs (esp. /var/log/syslog*) might help here too. Then you have to verify that the drive that went offline isn't broken (logs, again, and SMART).

Then kill the other raid (mdadm --fail, mdadm --remove) and add the device to the correct one. This will trigger a resync, but then you should have a fine raid again.

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The disks/partitions that are showing "U" are the ones that will be most up-to-date. There is no need to check the now out-of-date disks as mdadm knows which one is most current. – Alastair McCormack Nov 8 '12 at 8:51

You should just be able to re-add the disk with the -a option. For example:

mdadm -a /dev/md127 /dev/sdc1

Use mdadm -D /dev/mddevice to get more information about the array. You may need to use -r option first to remove the problematic disk partition before re-adding

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I had to do mdadm --stop /dev/md126 before adding it again – Claes Mogren Feb 9 '13 at 15:15

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