GRUB, which stands for Grand Unified Bootloader, is the program that is responsible for booting Linux Mint. If, for some reason, it becomes damaged, you won't be able to boot your machine and will get errors. This could be for any number of reasons, such as Windows overwriting the Master Boot Record (MBR), a failed cloning attempt, or even nonfailure incidents such as simply wanting to make something bootable, such as a flash drive.

If you want to install GRUB on a different disk, the grub-install command, shown in the following command line, should be all that you need:

sudo grub-install <device>

For <device>, simply type the device on which you're attempting to install GRUB. If you want to reinstall GRUB on your main drive, <device> will typically be /dev/sda, but you should always verify your devices before installing or reinstalling GRUB. The following command will list your drives:

sudo fdisk -l