How to restore MBR from backup in Linux

MBR or Master Boot Record is the first section of a disk responsible for loading bootloaders such as GRUB. It enables your system to boot into an operating system; thus, keeping a backup of your MBR is essential. On the other hand, GRUB can be re-generated if lost or deleted.

Master Boot Record can be restored using dd. It is a command-line tool also used to create the backup. If you cannot boot into your Linux system, you will need to boot up from a secondary system like an Ubuntu installer disk. From there, you can load up your MBR backup to restore.

1. Boot into Linux using Live CD / USB Drive if you can't boot into your system.
2. Launch terminal.
3. Check if your MBR backup is correct.

   $ 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. List available disks and identify the one with the corrupted master boot record.

   $ 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 master boot record of the disk from the backup using dd.

   $ 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. Check if your MBR is correctly restored.

   $ 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. Reboot your system to test.