How to restore Master Boot Record (MBR) in Linux

The Master Boot Record (MBR) is the initial section of a disk that handles bootloaders, such as GRUB, to enable your system to boot into an operating system. It's crucial to maintain an MBR backup, as GRUB can be re-generated if lost or deleted.

To restore the MBR, you'll need to use dd, a command-line tool that can also create backups. If your Linux system is unbootable, you must use a secondary system, such as an Ubuntu installer disk, to load and restore your MBR backup.

1. If your system won't boot, use a Live CD or USB drive to access Linux.
2. Open the terminal.
3. Verify that 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 the available disks and find the one with the damaged 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. Use dd to restore the disk's master boot record from the backup.

   $ 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. Confirm that your MBR has been properly 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. Restart your system to ensure it's functioning correctly.