How to repair/re-install GRUB2 from LiveUSB when using BTRFS subvolumes

[oshunluvr]

I had a fresh KDEneon install to a VM that failed to install GRUB initially for some unknown reason. So I booted back to the LiveUSB and mounted the virtual drive and sure enough - no grub files present in /boot/grub. So I tried to reinstall GRUB from the LiveUSB session and had a heck of a time until I realized the errors I was getting in step 4 - file not found, operation not permitted, etc. - were due to root being in a subvolume. I had mounted the whole file system instead.

I decided to write this up in case anyone else runs into this. This How-To was tested on KDEneon in a virtual machine.

For this How To:

• The boot drive is /dev/sda
• EFI (if applicable) is on “/dev/sda1”
• The BTRFS root file system is on “/dev/sda2”
• The root subvolume is “@”

Adjust the device names and/or subvolume name in the instructions accordingly for your installation.

Steps:

1. Boot to a LiveUSB
2. Mount the root file system

sudo mount -o subvol=@ /dev/sda2 /mnt

Note the use of the mount option “-o subvol=@”. This is the critical difference between a BTRFS installation vs. other file systems. The default for *buntu installations is to use "@" as the root subvolume.

3. Mount the folders for the chroot:

for i in dev dev/pts sys proc run; do sudo mount --bind /$i /mnt/$i; done

If you are using UEFI, you should mount the EFI boot partition as well:

sudo mount /dev/sda1 mnt/boot/efi

4. “chroot” root and reinstall GRUB

sudo chroot /mnt

5. Reinstall and update the GRUB bootloader:

sudo grub-install /dev/sda
sudo update-grub

6. Leave chroot and shutdown:

exit
poweroff

You may now remove the LiveUSB and power up again. Your install should now boot correctly.