I have a RAID 10 array consisting of 4 disks. One of them has failed and I have removed it from the array.

I have purchased an identical new drive and am unsure about how to add the new drive to the array.

In some literature that I read, it was mentioned that I need to copy the partitions to the new drive. In other online material that I read it seemed like the drive was simply added to the array and the syncing began without any mention of partitions having to be copied.

So now I am unsure how to proceed.

Here is some info about my raid 10 array.

cat /proc/mdstat:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md127 : active raid10 sda1[0] sdb1[1] sdc1[2]
      5860267008 blocks super 1.2 512K chunks 2 near-copies [4/3] [UUU_]

unused devices: <none>

sudo mdadm --detail /dev/md127

mdadm: unreconised word on DEVICE line: UUID=8eac0a3d-a22c-1a92-6ed9-f147f3dea64f
mdadm: unreconised word on DEVICE line: UUID=246226d0-a621-7f34-87b6-ff33b5c55906
mdadm: unreconised word on DEVICE line: UUID=52e1afa6-3ca6-a96e-f3dc-99bd0f56e878
mdadm: unreconised word on DEVICE line: UUID=16843e03-f177-97ef-07a7-6d761de9e99b
        Version : 1.2
  Creation Time : Sat Dec 13 15:01:15 2014
     Raid Level : raid10
     Array Size : 5860267008 (5588.79 GiB 6000.91 GB)
  Used Dev Size : 2930133504 (2794.39 GiB 3000.46 GB)
   Raid Devices : 4
  Total Devices : 3
    Persistence : Superblock is persistent

    Update Time : Mon Dec 31 22:07:21 2018
          State : clean, degraded
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

         Layout : near=2
     Chunk Size : 512K

           Name : server:0  (local to host server)
           UUID : 47c17420:8c4f5bf8:d70c8436:38187fe6
         Events : 973

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
       2       8       33        2      active sync   /dev/sdc1
       3       0        0        3      removed

gdisk -l of disk /dev/sda1 (in order to copy to new disk dev/sdd):

GPT fdisk (gdisk) version 0.8.8

Partition table scan:
  MBR: not present
  BSD: not present
  APM: not present
  GPT: not present

Creating new GPT entries.
Disk /dev/sda1: 5860530176 sectors, 2.7 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): AF72F8BD-75D0-469A-A7D3-3A4A40EFFFBE
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 5860530142
Partitions will be aligned on 2048-sector boundaries
Total free space is 5860530109 sectors (2.7 TiB)

Number  Start (sector)    End (sector)  Size       Code  Name

I would be very thankful for steps required to swap the failed drive.


This is a very straightforward and low-risk procedure: Assuming you have physically replace the drive, you need to

  • First of all, find the device name for the new disk: it should be something like /dev/sdX. To find it out, try cat /proc/partitions to identify the drive, that has exactly the same size as /dev/sda, /dev/sdb and /dev/sdc

  • Next we need to create the partition table. The easiest way is to run gdisk -l /dev/sda and note the exact values for /dev/sda1, then run gdisk /dev/sdX (with X the letter you identified before) and create an identical partition (if needed, you must apt-get install gdisk before)

  • Finally add the newly created partition to the array: mdadm --add /dev/md127 /dev/sdX1 (again with X as identified before)

When this is done, cat /proc/mdstat will show you the progress of the new disk being synchronized into the array.

  • Thanks so far. I have identified the new drive as /dev/sdd. I have run gdisk -l on one of the existing drives /dev/sda1. Not sure how to proceed with gdisk. Could it be that the existing drives in the array do not have a partition table? – PartisanEntity Jan 1 at 0:30
  • I figured it out. To copy partition table from existing drive to the new one and then randomize GUID: 'sgdisk /dev/sdX -R /dev/sdY' then sgdisk -G '/dev/sdY'. – PartisanEntity Jan 1 at 0:42
  • You needed to run gdisk -l on /dev/sda, not on /dev/sda1 ! – Eugen Rieck Jan 1 at 15:39

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