How to restore the master boot record (MBR) in Linux

The Master Boot Record (MBR) is an essential part of your disk that contains information necessary to boot your operating system. It stores the bootloader, like GRUB, which allows your system to start up. If the MBR becomes corrupted or is overwritten, your system might fail to boot. Knowing how to restore the MBR from a backup is critical to ensuring the system can function again.

Tools like dd in Linux can help restore the MBR, provided you have a valid backup. Restoring the MBR requires using a Live USB or installation media, such as an Ubuntu disk, to access the unbootable system. Identifying the correct disk and verifying the backup are important steps to avoid further damage.

Once the MBR is successfully restored, the bootloader will resume its function, allowing the system to boot normally. Using the correct procedure minimizes the risk of data loss and ensures the system returns to a bootable state.

1. Boot from a Live CD or USB to access your unbootable Linux system.
2. Open the terminal.
3. Verify the correctness of your MBR backup file.

   $ file mbr.bak 
   mbr.bak: DOS executable (COM)
   $ cat mbr.bak 
   �c������t��pt���y|1��؎м ��d|<�t��R��t��}��|�A��U�ZRr=��U�u7��t21��D@�D��D�f�\|f�f�`|f�\
   Z������}�f�ƈd�@f�D�������@�����f�f�`|f	�uNf�\|f1�f�4��1�f�t;}7����0�����Z�ƻp��1۸�r��`���1�������a�&Z|��}���}�4��}�.���GRUB GeomHard DiskRead Error
   ����<u��

4. Identify the disk with the damaged MBR from disk listing.

   $ lsblk
   NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
   loop0    7:0    0  55.4M  1 loop /snap/core18/1932
   loop1    7:1    0  55.4M  1 loop /snap/core18/1944
   loop2    7:2    0 217.9M  1 loop /snap/gnome-3-34-1804/60
   loop3    7:3    0   219M  1 loop /snap/gnome-3-34-1804/66
   loop4    7:4    0    51M  1 loop /snap/snap-store/498
   loop5    7:5    0  62.1M  1 loop /snap/gtk-common-themes/1506
   loop6    7:6    0    51M  1 loop /snap/snap-store/518
   loop7    7:7    0  64.8M  1 loop /snap/gtk-common-themes/1514
   loop8    7:8    0  31.1M  1 loop /snap/snapd/10492
   loop9    7:9    0  31.1M  1 loop /snap/snapd/10707
   sda      8:0    0    20G  0 disk 
   ├─sda1   8:1    0     1M  0 part 
   ├─sda2   8:2    0   513M  0 part /boot/efi
   └─sda3   8:3    0  19.5G  0 part /
   sr0     11:0    1  1024M  0 rom

5. Restore the MBR using dd to write from the backup file.

   $ sudo dd if=mbr.bak of=/dev/sda
   [sudo] password for user: 
   0+1 records in
   0+1 records out
   446 bytes copied, 0.00284831 s, 157 kB/s

6. Verify that the MBR was restored successfully.

   $ sudo head -c 446 /dev/sda
   �c������t��pt���y|1��؎м ��d|<�t��R��t��}��|�A��U�ZRr=��U�u7��t21��D@�D��D�f�\|f�f�`|f�\
   Z������}�f�ƈd�@f�D�������@�����f�f�`|f	�uNf�\|f1�f�4��1�f�t;}7����0�����Z�ƻp��1۸�r��`���1�������a�&Z|��}���}�4��}�.���GRUB GeomHard DiskRead Error
   ����<u��

7. Restart the system to check if it boots correctly.