Reconfigure GRUB from LiveCD

By Lye Heng Foo on February 13, 2021

There are times when you have a dual boot computer to boot into both Windows & Ubuntu (Linux).

And if you have to re-install Windows, chances are that Windows installation will overwrite any existing GRUB installation, and make your Ubuntu not bootable anymore.

To solve this, prepare a LiveCD for the same version of Ubuntu (preferably), so that you have the same GRUB bootloader version installed again. To prepare a LiveCD USB, you can download the ISO file and use any ISO image to USB writer app. Some good examples of this writer app are Universal USB Installer (UUI), Rufus and UNetbootin.

If you wish to create a few flavours of Linux ISOs in the same pendrive, you could use YUMI or MultiBootUSB.

After you have prepared the LiveCD, boot your computer into the LiveCD. And here are the commands you need to use to re-configure GRUB after booting into Live CD. Please take note that the example below is assuming /dev/sda2 is your Ubuntu/Linux partition (where the GRUB boot configuration is stored), and Windows is installed in /dev/sda1 and that /dev/sda is the primary boot disk. If you have something different, please replace accordingly. You can easily find out about your partition from fdisk -l (Linux) or simply use tools like GParted (Linux) or Minitool Partition Wizard (Windows)

fdisk -l   ### To find out about your partitions information

sudo mount /dev/sda2 /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /sys /mnt/sys
sudo mount --bind /proc /mnt/proc
sudo mount /dev/sdaX /mnt/boot   ### Only if /boot is mounted as separate partition

sudo chroot /mnt
sudo update-grub
sudo grub-install /dev/sda
exit

sudo umount /mnt/dev
sudo umount /mnt/sys
sudo umount /mnt/proc
sudo umount /mnt/boot   ### Only if you mounted it earlier

sudo reboot

Alternate fix:

sudo mount /dev/sda2 /mnt
sudo grub-install --root-directory=/mnt /dev/sda
sudo umount /dev/sda2
sudo reboot

You should be able to see the GRUB boot menu upon reboot. To be sure that everything is working (especially when you have more than one OS installed), please re-run OS detection by running:

sudo update-grub

Hope it has helped you.

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