Replace a disk in a RAID

Last update: May 30 2026

1. Remove the disk from the array.
2. Determine the disk partition scheme.
3. Copy the partition table to the new disk.
4. Add the disk to the RAID array.
5. Install the bootloader.

1. Remove the disk from the array

1. Connect to the server via SSH or via KVM console.

2. Output current RAID array configuration:

   cat /proc/mdstat

   The output will contain information about the RAID array configuration. Example:

   Personalities : [raid1]
   md1 : active raid1 sda3[0] sdb3[1]
       975628288 blocks super 1.2 [2/2] [UU]
       bitmap: 3/8 pages [12KB], 65536KB chunk
   
   md0 : active raid1 sda2[2] sdb2[1]
       999872 blocks super 1.2 [2/2] [UU]
   
   unused devices: <none>

   Where:

   • md0 — name of the RAID array assembled from the sda2 and sdb2;
   • md1 — name of the RAID array assembled from the sda3 and sdb3.

3. Output information about partitions on the disks and RAID array configuration:

   lsblk

   The output will contain a list of disks with partitions. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0     1M  0 part
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sda3             8:3    0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /
   sdb                8:16   0 931.5G  0 disk
   ├─sdb1             8:17   0     1M  0 part
   ├─sdb2             8:18   0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sdb3             8:19   0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /

   Where:

   • sda — name of the disk with functioning RAID arrays;
   • sdb — name of the disk to be replaced;
   • sdb2 — name of the partition on disk sdb that is a component of the first RAID array;
   • md0 — name of the first RAID array assembled from the sda2 and sdb2;
   • sdb3 — name of the partition on disk sdb that is a component of the second RAID array;
   • md1 — name of the second RAID array assembled from the sda3 and sdb3.

4. Mark the disk you need to replace as faulty:

   mdadm /dev/<raid_array_1> -f /dev/<partition_1>
   mdadm /dev/<raid_array_2> -f /dev/<partition_2>

   Specify:

   • <raid_array_1> — name of the first RAID array that you obtained in step 3; in this example, it is md0;
   • <partition_1> — name of the first RAID array partition on the replacement disk that you obtained in step 3; in this example, it is sdb2;
   • <raid_array_2> — name of the second RAID array that you obtained in step 3; in this example, it is md1;
   • <partition_2> — name of the second RAID array partition on the replacement disk that you obtained in step 3; in this example, it is sdb3.

5. Remove the disk you marked as faulty in step 4 from all devices:

   mdadm /dev/<raid_array_1> --remove /dev/<partition_1>
   mdadm /dev/<raid_array_2> --remove /dev/<partition_2>

   Specify:

   • <raid_array_1> — name of the first RAID array that you obtained in step 3; in this example, it is md0;
   • <partition_1> — name of the first RAID array partition on the replacement disk that you obtained in step 3; in this example, it is sdb2;
   • <raid_array_2> — name of the second RAID array that you obtained in step 3; in this example, it is md1;
   • <partition_2> — name of the second RAID array partition on the replacement disk that you obtained in step 3; in this example, it is sdb3.

2. Determine the disk partition scheme

1. Connect to the server via SSH or via KVM console.

2. Install the utility for working with the GPT (GUID Partition Table) on disks — gdisk:

   apt-get install gdisk -y

3. Output information about partitions on the disks:

   lsblk

   The output will contain information about partitions on the disks. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0     1M  0 part
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sda3             8:3    0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /
   sdb                8:16   0 931.5G  0 disk

   Where:

   • sda — name of the disk with functioning RAID partitions;
   • sdb — name of the new disk.

4. Determine the disk partition scheme:

   gdisk -l /dev/<source_disk>

   Specify <source_disk> — name of the disk with functioning RAID partitions that you obtained in step 3; in this example, it is sda.

   The answer depends on the disk partition scheme:

   • GPT:

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

   • MBR:

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

3. Copy the partition table to the new disk

The partition table copying process depends on the disk partition scheme — GPT or MBR.

carefully

When copying the partition table, it is essential to specify the disks in the correct order in the command. If the disk order is incorrect, the healthy disk will be overwritten, and data will be deleted. Data cannot be recovered.

GPT

1. Connect to the server via SSH or via KVM console.

2. Output information about partitions on the disks:

   lsblk

   The output will contain information about partitions on the disks. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0     1M  0 part
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sda3             8:3    0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /
   sdb                8:16   0 931.5G  0 disk

   Where:

   • sda — name of the disk with functioning RAID partitions;
   • sdb — name of the new disk.

3. Copy the partition table to the new disk. It is essential to specify the disks in the correct order in the command; otherwise, the partition table and data on the originally healthy disk will be destroyed.

   sgdisk -R /dev/<new_disk> /dev/<source_disk>

   Specify:

   • <new_disk> — name of the new disk to which the partition layout is copied; you obtained it in step 2; in this example, it is sdb;
   • <source_disk> — name of the disk with functioning RAID partitions from which the partition layout is copied; you obtained it in step 2; in this example, it is sda.

4. Assign a random UUID to the new disk:

   sgdisk -G /dev/<new_disk>

   Specify <new_disk> — name of the new disk to which the partition layout was copied; you obtained it in step 2; in this example, it is sdb.

5. Ensure the partition schemes on the disks are identical:

   lsblk

   The output will contain information about partitions on the disks. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0     1M  0 part
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sda3             8:3    0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /
   sdb                8:16   0 931.5G  0 disk
   ├─sdb1             8:17   0     1M  0 part
   ├─sdb2             8:18   0   977M  0 part
   └─sdb3             8:19   0 930.6G  0 part

MBR

1. Connect to the server via SSH or via KVM console.

2. Output information about partitions on the disks:

   lsblk

   The output will contain information about partitions on the disks. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0     1M  0 part
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sda3             8:3    0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /
   sdb                8:16   0 931.5G  0 disk

   Where:

   • sda — name of the disk with functioning RAID partitions;
   • sdb — name of the new disk.

3. Copy the partition table to the new disk. It is essential to specify the disks in the correct order in the command; otherwise, the partition table and data on the originally healthy disk will be destroyed.

   sfdisk -d /dev/<source_disk> | sfdisk /dev/<new_disk>

   Specify:

   • <source_disk> — name of the disk with functioning RAID partitions from which the partition layout is copied; you obtained it in step 2; in this example, it is sda;
   • <new_disk> — name of the new disk to which the partition layout is copied; you obtained it in step 2; in this example, it is sdb.

4. If partitions are not visible in the system, refresh partition information:

   partprobe /dev/<new_disk>

   Specify <new_disk> — name of the new disk to which the partition layout was copied; you obtained it in step 2; in this example, it is sdb.

5. Ensure the partition schemes on the disks are identical:

   lsblk

   The output will contain information about partitions on the disks. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0     1M  0 part
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sda3             8:3    0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /
   sdb                8:16   0 931.5G  0 disk
   ├─sdb1             8:17   0     1M  0 part
   ├─sdb2             8:18   0   977M  0 part
   └─sdb3             8:19   0 930.6G  0 part

4. Add the disk to the RAID array

1. Connect to the server via SSH or via KVM console.

2. Output information about partitions on the disks and RAID array configuration:

   lsblk

   The output will contain information about partitions on the disks and RAID array configuration. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0     1M  0 part
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sda3             8:3    0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /
   sdb                8:16   0 931.5G  0 disk
   ├─sdb1             8:17   0     1M  0 part
   ├─sdb2             8:18   0   977M  0 part
   └─sdb3             8:19   0 930.6G  0 part

   Where:

   • md0 — name of the first RAID array which is assembled from the sda2;
   • md1 — name of the second RAID array which is assembled from the sda3;
   • sdb2 — name of the first partition on the new disk to be added to the RAID array;
   • sdb3 — name of the second partition on the new disk to be added to the RAID array.

3. Add the partitions of the new disk to the RAID array:

   mdadm /dev/<raid_array_1> -a /dev/<partition_1>
   mdadm /dev/<raid_array_2> -a /dev/<partition_2>

   Specify:

   • <raid_array_1> — name of the first RAID array that you obtained in step 2; in this example, it is md0;
   • <partition_1> — name of the first partition on the new disk that you obtained in step 2; in this example, it is sdb2;
   • <raid_array_2> — name of the second RAID array that you obtained in step 2; in this example, it is md1;
   • <partition_2> — name of the second partition on the new disk that you obtained in step 2; in this example, it is sdb3.

   After adding a disk to a RAID array, synchronization will start. The synchronization speed depends on the size and type of the disk — SSD or HDD.

4. Wait for synchronization to complete.

5. Ensure that the new disk has been added to the RAID array:

   lsblk

   The output will contain information about partitions on the disks and RAID array configuration. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0     1M  0 part
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sda3             8:3    0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /
   sdb                8:16   0 931.5G  0 disk
   ├─sdb1             8:17   0     1M  0 part
   ├─sdb2             8:18   0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1
   └─sdb3             8:19   0 930.6G  0 part
     └─md1            9:1    0 930.4G  0 raid1  /

5. Install the bootloader

After adding the disk to the array, you need to install the OS bootloader on it.

You can install the bootloader both in the OS and in Rescue mode.

Server booted into the OS

1. Connect to the server via SSH or via KVM console.

2. Output information about partitions on the disks:

   lsblk -o +FSTYPE

   The output will contain information about partitions on the disks. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT  FSTYPE
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0   500M  0 part              vfat
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1             ext4
   └─sda3             8:3    0 930.1G  0 part
     └─md1            9:1    0 929.9G  0 raid1  / 
   sdb                8:16   0 931.5G  0 disk
   ├─sdb1             8:17   0   500M  0 part          
   ├─sdb2             8:18   0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1             ext4
   └─sdb3             8:19   0 930.1G  0 part
     └─md1            9:1    0 929.9G  0 raid1  /          ext4 

   Where:

   • sda — name of the disk with functioning RAID arrays;
   • sdb — name of the new disk.

3. Install the GRUB bootloader:

   grub-install /dev/<new_disk>

   Specify <new_disk> — name of the new disk that you obtained in step 2; in this example, it is sdb.

Server booted in Rescue mode

1. Boot the server into Rescue emergency and recovery mode.

2. Connect to the server via SSH or via KVM console.

3. Output information about the OS boot mode:

   [ -d /sys/firmware/efi ] && echo "UEFI" || echo "BIOS"

   The output will contain information about the OS boot mode — BIOS or UEFI.

4. Check if the system has detected RAID arrays by outputting information about partitions on the disks:

   lsblk -o +FSTYPE

   The output will contain information about partitions on the disks and RAID array configuration. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT  FSTYPE
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0   500M  0 part              vfat
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1             ext4
   └─sda3             8:3    0 930.1G  0 part
     └─md1            9:1    0 929.9G  0 raid1  
   sdb                8:16   0 931.5G  0 disk
   ├─sdb1             8:17   0   500M  0 part          
   ├─sdb2             8:18   0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1             ext4
   └─sdb3             8:19   0 930.1G  0 part
     └─md1            9:1    0 929.9G  0 raid1             ext4 

   Here md0, md1 are the names of the RAID arrays on the disk partitions.

5. If there are RAID arrays in the lsblk output after step 4, skip this step. If there are no RAID arrays, start assembling the RAID arrays:

   mdadm --assemble --scan

6. Output information about partitions on the disks and RAID array configuration:

   lsblk -o +FSTYPE

   The output will contain information about partitions on the disks and RAID array configuration. Example:

   NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT  FSTYPE
   loop0              7:0    0   985M  1 loop
   sda                8:0    0 931.5G  0 disk
   ├─sda1             8:1    0   500M  0 part              vfat
   ├─sda2             8:2    0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1             ext4
   └─sda3             8:3    0 930.1G  0 part
     └─md1            9:1    0 929.9G  0 raid1  
   sdb                8:16   0 931.5G  0 disk
   ├─sdb1             8:17   0   500M  0 part          
   ├─sdb2             8:18   0   977M  0 part
   │ └─md0            9:0    0 976.4M  0 raid1             ext4
   └─sdb3             8:19   0 930.1G  0 part
     └─md1            9:1    0 929.9G  0 raid1             ext4 

   Where:

   • sda — name of the disk with functioning RAID partitions;
   • sdb — name of the new disk;
   • md0, md1 are the names of the RAID arrays on the disk partitions.

7. Identify the partitions to be mounted:

   • root partition / — usually located on the largest partition of the disk; in the example in step 6, it is md1;
   • boot partition /boot — usually has the ext4 file system and a size of 512 MB to 1 GB; in the example in step 6, it is md0;
   • EFI partition /boot/efi — a partition with the vfat file system, used when booting the OS in UEFI mode; in the example in step 6, these are partitions sda1 and sdb1.

8. Mount the root file system to the /mnt directory:

   mount /dev/<raid_array> /mnt

   Here, <system_raid_array> is the name of the root partition that you identified in step 7; in this example, it is md1.

9. Mount the boot partition:

   mount /dev/<boot_partition> /mnt/boot

   Specify <boot_raid_array> — name of the boot partition /boot that you identified in step 7; in this example, it is md0.

10. If the OS uses a UEFI bootloader, mount the EFI partition:

    mount /dev/<efi_partition> /mnt/boot/efi

    Specify <efi_partition> — name of the EFI partition /boot/efi on the new disk that you identified in step 7; in this example, it is sdb1.

11. Mount the service file systems:

    mount --bind /sys /mnt/sys
    mount --bind /proc /mnt/proc
    mount --bind /dev /mnt/dev
    mount -t devpts devpts /mnt/dev/pts

12. If you identified the OS boot mode as UEFI in step 3, mount the efivars file system:

mount -t efivarfs efivarfs /mnt/sys/firmware/efi/efivars

13. Connect to the environment:

    chroot /mnt /bin/bash

14. Export the PATH variable:

    export PATH=/bin:/usr/bin:/usr/local/bin:/sbin:/usr/sbin:/usr/local/sbin

15. Install the GRUB bootloader. The command to install it depends on the OS boot mode that you identified in step 3:

    • UEFI:

    grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=GRUB

    • BIOS:

    grub-install /dev/<new_disk>

    Specify <new_disk> — name of the new disk that you obtained in step 6; in this example, it is sdb.

16. Create the GRUB configuration file:

    /sbin/grub-mkconfig -o /boot/grub/grub.cfg

17. After the GRUB installation is finished, exit the environment:

    exit

18. Unmount the service file systems:

    mount -t devpts devpts /mnt/dev/pts
    mount --bind /dev /mnt/dev
    mount --bind /proc /mnt/proc
    mount --bind /sys /mnt/sys

19. Unmount the file system:

    umount /dev/<system_raid_array> /mnt

    Specify <system_raid_array> — name of the root partition / that you mounted in step 8; in this example, it is md1.

20. Return to the previous server boot template or reboot the server from the OS. When rebooting the server from the OS, the boot template will automatically change to the one that was set before booting the server in Rescue mode.