Replacing a disk in a RAID array

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Updated at July 7, 2025

Remove the failed disk from the RAID array
Request physical disk replacement
Add the new disk to your RAID array

If a disk in a BareMetal server’s RAID array fails, stop using it, remove it from the array, request a replacement drive from support, and add the new disk to the array.

Note

This guide does not apply to disk failures in RAID 0 arrays. Such arrays are not fault-tolerant; a single disk failure results in complete data loss and requires full array reconstruction.

This guide covers a standard RAID10 configuration with four HDDs under Ubuntu 24.04. If your setup differs from this standard configuration, adjust the following steps accordingly.

Remove the failed disk from the RAID array

Connect to the server over SSH:
```
ssh root@<server_public_IP_address>
```
You can also access the server via the KVM console using your username and password.

Check the current disk and partition layout in the RAID array:

cat /proc/mdstat

Result:

Personalities : [raid10] [raid0] [raid1] [raid6] [raid5] [raid4]
md3 : active raid10 sdb4[1] sdc4[2] sdd4[3] sda4[0]
      3893569536 blocks super 1.2 256K chunks 2 near-copies [4/4] [UUUU]
      bitmap: 0/30 pages [0KB], 65536KB chunk

md2 : active raid10 sdc3[2] sdb3[1] sdd3[3] sda3[0]
      2095104 blocks super 1.2 256K chunks 2 near-copies [4/4] [UUUU]

md1 : active raid10 sdc2[2] sdb2[1](F) sda2[0] sdd2[3]
      8380416 blocks super 1.2 256K chunks 2 near-copies [4/3] [U_UU]

As shown, the RAID array consists of three RAID partitions: md1 composed of sdb2 and sda2 disk partitions (on physical disks sdb and sda, respectively), md2 composed of sdb3 and sda3 disk partitions, and md3 composed of sdb4 and sda4 disk partitions. The command output shows that the sdb disk has failed, indicated by the (F) flag next to its name.

Additionally, you can check the role of each partition in the RAID array:

lsblk

Result:

NAME    MAJ:MIN RM  SIZE RO TYPE   MOUNTPOINTS
sda       8:0    0  1.8T  0 disk
├─sda1    8:1    0  299M  0 part
├─sda2    8:2    0    4G  0 part
│ └─md1   9:1    0    8G  0 raid10 /boot
├─sda3    8:3    0    1G  0 part
│ └─md2   9:2    0    2G  0 raid10 [SWAP]
└─sda4    8:4    0  1.8T  0 part
  └─md3   9:3    0  3.6T  0 raid10 /
sdb       8:16   0  1.8T  0 disk
├─sdb1    8:17   0  299M  0 part
├─sdb2    8:18   0    4G  0 part
│ └─md1   9:1    0    8G  0 raid10 /boot
├─sdb3    8:19   0    1G  0 part
│ └─md2   9:2    0    2G  0 raid10 [SWAP]`
└─sdb4    8:20   0  1.8T  0 part
  └─md3   9:3    0  3.6T  0 raid10 /
sdc       8:32   0  1.8T  0 disk
├─sdc1    8:33   0  299M  0 part
├─sdc2    8:34   0    4G  0 part
│ └─md1   9:1    0    8G  0 raid10 /boot
├─sdc3    8:35   0    1G  0 part
│ └─md2   9:2    0    2G  0 raid10 [SWAP]
└─sdc4    8:36   0  1.8T  0 part
  └─md3   9:3    0  3.6T  0 raid10 /
sdd       8:48   0  1.8T  0 disk
├─sdd1    8:49   0  299M  0 part
├─sdd2    8:50   0    4G  0 part
│ └─md1   9:1    0    8G  0 raid10 /boot
├─sdd3    8:51   0    1G  0 part
│ └─md2   9:2    0    2G  0 raid10 [SWAP]
└─sdd4    8:52   0  1.8T  0 part
  └─md3   9:3    0  3.6T  0 raid10 /

In our example:

md1: /boot partition.
md2: SWAP partition.
md3: / root partition.

Assume the /dev/sdb disk has failed. Detach the /dev/sdb disk's partitions from the RAID array:

mdadm /dev/md1 --remove /dev/sdb2
mdadm /dev/md2 --remove /dev/sdb3
mdadm /dev/md3 --remove /dev/sdb4

The mdadm utility prevents disk removal from a RAID array if the disk is still operational or if removal can cause array failure, triggering a Device busy error:

mdadm: hot remove failed for /dev/sdb2: Device or resource busy

In this case, first mark the disk as failed before retrying the removal:

mdadm /dev/md1 --fail /dev/sdb2
mdadm /dev/md1 --remove /dev/sdb2
mdadm /dev/md2 --fail /dev/sdb3
mdadm /dev/md2 --remove /dev/sdb3
mdadm /dev/md3 --fail /dev/sdb4
mdadm /dev/md3 --remove /dev/sdb4

Get the failed disk's ID:

fdisk -l

Result:

...
Disk /dev/sdb: 838.36 GiB, 900185481216 bytes, 1758174768 sectors
Disk model: SAMSUNG MZ7GE900
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: CD2ACB4C-1618-4BAF-A6BB-D2B9********
...

Save the Disk identifier value for your support ticket.

Request physical disk replacement

Submit a disk replacement ticket to technical support, including your BareMetal server ID and failed disk ID.

Wait for data center engineers to replace the failed disk.

Add the new disk to your RAID array

After the physical drive replacement, partition the new disk and add it to the existing RAID array.

Use the gdisk utility to specify the partition table type: GPT or MBR. If needed, install gdisk on your server’s operating system.

Run the following command, specifying the ID of the remaining operational disk in the RAID array:
```
gdisk -l /dev/sda
```
Depending on the partition table type, the result will be as follows:
GPT

MBR
```
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present
...
```
```
Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: not present
...
```
Copy the partition table layout from the remaining operational disk in the RAID array to the new disk:
GPT

MBR
If the source disk uses a GPT partition table:
1. Create a copy of the source disk partition table:
  sgdisk --backup=table /dev/sda
  Result:
  The operation has completed successfully.
2. Restore the partition table from the backup copy to the new disk:
  sgdisk --load-backup=table /dev/sdb
  Result:
  The operation has completed successfully.
3. Assign a new random UUID to the new disk:
  sgdisk -G /dev/sdb
  Result:
  The operation has completed successfully.
If the source disk uses an MBR partition table:
1. Copy the partition table:
  sfdisk -d /dev/sda | sfdisk /dev/sdb
  Where:
  - /dev/sda: The remaining operational disk in the RAID array used as the partition table template.
  - /dev/sdb: The new disk that will receive the partition table copy from the source disk.
2. If new partitions are not visible after copying, reload the partition table:
  sfdisk -R /dev/sdb
Add the disk to the RAID array by sequentially adding each of its partitions to their corresponding RAID components. The correspondence between disk partitions and RAID components was described earlier in Remove the failed disk from the RAID array.

Run the following commands:
```
mdadm /dev/md1 --add /dev/sdb2
mdadm /dev/md2 --add /dev/sdb3
mdadm /dev/md3 --add /dev/sdb4
```
Once added to the array, the disk begins synchronizing. The sync speed depends on the disk capacity and type: ssd or hdd.

Result:
```
mdadm: added /dev/sdb2
mdadm: added /dev/sdb3
mdadm: added /dev/sdb4
```

Make sure the new disk has been successfully added to the RAID array:

cat /proc/mdstat

Result:

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 sdb3[2] sda3[0]
      6287360 blocks super 1.2 [2/2] [UU]

md3 : active raid1 sdb4[2] sda4[0]
      849215488 blocks super 1.2 [2/2] [UU]
      bitmap: 4/7 pages [16KB], 65536KB chunk

md1 : active raid1 sdb2[2] sda2[0]
      10477568 blocks super 1.2 [2/2] [UU]

unused devices: <none>

Install the Linux bootloader on the new disk:

grub-install /dev/sdb

Result:

Installing for i386-pc platform.
Installation finished. No error reported.

Replacing a disk in a RAID array

Remove the failed disk from the RAID array

Request physical disk replacement

Add the new disk to your RAID array

See also

Was the article helpful?

Replacing a disk in a RAID array

Remove the failed disk from the RAID arrayRemove the failed disk from the RAID array

Request physical disk replacementRequest physical disk replacement

Add the new disk to your RAID arrayAdd the new disk to your RAID array

See alsoSee also

Was the article helpful?

Remove the failed disk from the RAID array

Request physical disk replacement

Add the new disk to your RAID array

See also