repair-grub-live-usb.md

Repair / Install GRUB from live USB

Requirements

1. Already installed linux partition for which Grub needs to installed or repaired.
2. Patience and some terminal knowledge is helpful.

Procedure

Assuming linux is installed in /dev/sda3. This will be different for your system. Use Gparted program to check.

1. Boot from the USB drive. Open a terminal with "Alt+Ctrl+T".

   OS="/dev/sda3"
   EFI="/dev/sda1"
   MOUNT="/cdrom"
   

2. Mount the partition.

   sudo mount "$OS" "$MOUNT"
   

3. Bind mount some necessary partitions from live USB environment.

   sudo mount -t proc /proc "${MOUNT}/proc/"
   sudo mount --rbind /sys "${MOUNT}/sys/"
   sudo mount --rbind /dev "${MOUNT}/dev/"
   

4. Chroot into the mount point.

   sudo chroot "$MOUNT"
   

5. Now we are in the environment of the actual linux installation. Usually Grub is installed in /dev/sda1. Hence we need to mount it.

   sudo mount /dev/sda1 /mnt
   

   This /mnt is different than the one in the previous steps. This one belongs to the actual linux install.
6. Update grub and install.

   sudo grub-install --efi-directory=/mnt  /dev/sda
   # alternative
   # sudo grub-install --target=x86_64-efi --efi-directory=/mnt/ --bootloader-id=<some name>
   

7. Finally, unmount partitions. In chroot environment:

   sudo umount /mnt
   exit
   

   In live USB:

   sudo umount "${MOUNT}/proc"
   sudo umount "${MOUNT}/sys"
   sudo umount "${MOUNT}/dev"
   sudo umount "$MOUNT"
   

Reboot and enjoy.

Sources:
https://unix.stackexchange.com/questions/96977/grub-probe-error-failed-to-get-canonical-path-of-cow
https://superuser.com/questions/165116/mount-dev-proc-sys-in-a-chroot-environment