Dennis Núñez

PhD (c) in AI and Neuroimaging. CEA / Inria / Université Paris-Saclay

Repair or Reinstall GRUB2

Grub 2 typically gets overridden when you install Windows or another Operating System. To make Ubuntu control the boot process, you need Reinstall (Repair/Restore) Grub using a Ubuntu Live CD.


Using the sudo command, especially from a Live CD can do serious damage to your system. Read all instructions and confirm you understand before executing any commands. When pasting into the Terminal, use Ctrl+Shift+V, NOT Ctrl+V.

Terminal Commands

Mount the partition your Ubuntu Installation is on. If you are not sure which it is, launch GParted (included in the Live CD) and find out. It is usually a EXT4 Partition. Replace the XY with the drive letter, and partition number, for example: sudo mount /dev/sda3 /mnt.

$ sudo mount /dev/sdXY /mnt

Now bind the directories that grub needs access to to detect other operating systems, like so.

$ sudo mount --bind /dev /mnt/dev && $ sudo mount --bind /dev/pts /mnt/dev/pts && $ sudo mount --bind /proc /mnt/proc && $ sudo mount --bind /sys /mnt/sys

Now we jump into that using chroot.

$ sudo chroot /mnt

Now install, check, and update grub.

This time you only need to add the drive letter (usually a) to replace X, for example: grub-install /dev/sda, grub-install –recheck /dev/sda.

$ grub-install /dev/sdX $ grub-install --recheck /dev/sdX $ update-grub

Now grub is back, all that is left is to exit the chrooted system and unmount everything.

$ exit && $ sudo umount /mnt/sys && $ sudo umount /mnt/proc && $ sudo umount /mnt/dev/pts && $ sudo umount /mnt/dev && $ sudo umount /mnt

Shut down and turn your computer back on, and you will be met with the default Grub2 screen.

You may want to update grub or re-install burg however you like it.

Congratulations, you have just Repaired/Restored/Reinstalled Grub 2 with a Ubuntu Live CD!.

Ojo: If appear this warning: sector 32 is already in use by flexnet, write:

$ sudo dd if=/dev/zero of=/dev/sda bs=512 count=1 seek=32

Then, you shuld realise all steps again.