MBR or Master Boot Record is a small program that defines how operating systems are loaded. It resides in the first 446 bytes of your boot disk and could be deleted or replaced when installing a new operating systems such as Microsoft Windows, making you unable to boot into your other installed operating systems.
This makes it essential to backing up your MBR, especially prior to installing a new operating systems. MBR could be backed up via Linux using dd at the terminal.
$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT loop0 7:0 0 89M 1 loop /snap/core/7713 loop1 7:1 0 54.6M 1 loop /snap/lxd/11964 loop2 7:2 0 54.6M 1 loop /snap/lxd/11985 loop3 7:3 0 88.7M 1 loop /snap/core/7396 sda 8:0 0 20G 0 disk ├─sda1 8:1 0 1M 0 part └─sda2 8:2 0 20G 0 part / sr0 11:0 1 748M 0 rom
$ sudo dd if=/dev/sda of=/home/user/mbr.bak bs=446 count=1 [sudo] password for user: 1+0 records in 1+0 records out 446 bytes copied, 0.00140727 s, 317 kB/s
$ strings /home/user/mbr.bak ZRr= `|f \|f1 GRUB Geom Hard Disk Read Error
The file is in binary format but you should be able to see some strings such as GRUB (if you're using GRUB as your bootloader) as hint that you've copied the right section of the disk.
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