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I had a server with 2 disks in a raid and one of them failed. Called the provider and had them change the disk.

After a reboot I still see only one disk:

cat /proc/mdstat 
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md2 : active raid1 sda3[0]
      1073610560 blocks super 1.2 [2/1] [U_]

md3 : active raid1 sda4[0]
      1839089920 blocks super 1.2 [2/1] [U_]

md0 : active raid1 sda1[0]
      16768896 blocks super 1.2 [2/1] [U_]

md1 : active raid1 sda2[0]
      523968 blocks super 1.2 [2/1] [U_]

unused devices: <none>

And it looks like the new drive is removed:

mdadm -D /dev/md3
/dev/md3:
        Version : 1.2
  Creation Time : Wed Jun 17 00:26:21 2015
     Raid Level : raid1
     Array Size : 1839089920 (1753.89 GiB 1883.23 GB)
  Used Dev Size : 1839089920 (1753.89 GiB 1883.23 GB)
   Raid Devices : 2
  Total Devices : 1
    Persistence : Superblock is persistent

    Update Time : Sun Nov  5 15:56:00 2017
          State : clean, degraded 
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           Name : rescue:3
           UUID : 0c807ba7:4535e375:273f715a:7ab59c54
         Events : 2851

    Number   Major   Minor   RaidDevice State
       0       8        4        0      active sync   /dev/sda4
       2       0        0        2      removed

So the question is how do I enable my new disk in /dev/sdb? It looks OK when I test it with the tool:

smartctl -H /dev/sdb
smartctl 6.5 2016-01-24 r4214 [x86_64-linux-4.4.0-98-generic] (local build)
Copyright (C) 2002-16, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

How do I proceed?

1

First backup important data, the best complete partition with dd (when no program is writing there). You can use something like this:

dd if=/dev/md3 of=/your/backup/path/md3.dump bs=1M

So you're safe if something goes wrong...

https://www.howtoforge.com/replacing_hard_disks_in_a_raid1_array

  • Just the links answers the question. Making a backup is a good hint though. – Marco Nov 5 '17 at 17:29
  • 1
    Great links. The second one did the trick (synchronizing now anyway). Thanks a lot. – Theis Nov 5 '17 at 18:08

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