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Accepted
FalcoGer
Asked: 2023-04-17 23:41:19 +0800 CST

Fix GRUB2 UEFI bootloader MBR after moving /boot mountpoint

  • 772

I have installed a debian based distribution on my drive. When I installed it, the / mountpoint was btrfs on a logical partition (/dev/sda7) with a mount point for /boot/efi on a fat32 logical partition (/dev/sda6).

I since wanted to use the savedefault feature of grub, which doesn't work on btrfs. So I booted up a live image with gparted and shrunk /boot/efi (/dev/sda6) added an ext4 partition /dev/sda8.

After that I adjusted /etc/fstab to use the new mount point and then followed the steps to repair grub.

  1. mount the root file system mkdir /tmp/rootfs && mount /dev/sda7 /tmp/rootfs
  2. mount the boot drive on the mounted file system mount /dev/sda8 /tmp/rootfs/@/boot
  3. mount the efi partition mount /dev/sda6 /tmp/rootfs/@/boot/efi
  4. mount sys mount --bind /sys /tmp/rootfs/@/sys
  5. mount dev mount --bind /dev /tmp/rootfs/@/dev
  6. mount proc mount --bind /proc /tmp/rootfs/@/proc
  7. for good measure mount home mount --bind /tmp/rootfs/@home /tmp/rootfs/@/home
  8. chroot into the rootfs chroot /tmp/rootfs
  9. repair grub, apparently

To repair grub I used this command:

grub-install /dev/sda

And it says it's successful. However when I then try to boot the system I get this error:

GRUB loading...
Welcome to GRUB!

error: file `/@/boot/grub/i386-pc/normal.mod´ not found.
grub rescue>

Of course it can't find it there because it's not in /@/boot/ anymore. I think this is an error with the stage 1 bootloader in the MBR. How do I fix that? I use an msdos partition table.

I have also managed to boot the system from grub loaded from a USB drive with this command

set root=(hd1,msdos8)
linux /vmlinuz root=LABEL=LinuxRoot rootflags=subvol=@ quiet splash rw
initrd /initrd
boot

I have tried to reinstall grub from there as well, with no success. I have also tried this command, which also didn't work (says successful, but same error appears on reboot).

grub-install --bootloader-id=grub --target=x86_64-efi --efi-directory=/boot/efi --no-nvram /dev/sda
boot

2 Answers

  1. Best Answer

    I was able to resolve this properly by converting my msdos partition table to a gpt partition table. I did that by booting a live cd and running

    gdisk /dev/sdX
w

    Afterwards I had to reinstall the windows bootloader with a live medium and open a command prompt. Then run this sequence of commands to install a new bootloader to the EFI partition.

    diskpart
  list disk
  select disk <volume for EFI partition>
  assign letter=O
  exit
bcdboot D:\Windows /s O
O:
cd EFI
mkdir Microsoft
cd Microsoft
bootrec /Fixboot

    After that I just had to reinstall grub (target x86_64-efi). os-probe didn't detect windows, so I had to add it manually by adding it a new file /etc/grub.d/42-windows file like so

    cat<<EOF
menuentry Windows {
    search --no-floppy --set=root --file '/EFI/Microsoft/Boot/bootmgfw.efi'
    chainloader /EFI/Microsoft/Boot/bootmgfw.efi
}
EOF

    After that run sudo update-grub and everything works fine.

    • 0

  2. I was able to fix it by booting into the system as described above and then running

    sudo apt install grub-pc-bin
sudo grub-install --bootloader-id=grub --target=i386-pc /dev/sda
sudo update-grub
sudo reboot

    After reboot, run:

    sudo apt remove grub-pc-bin
sudo grub-install --bootloader-id=grub --target=x86_64-efi --efi-directory=/boot/efi /dev/sda # no-nvram is detected automatically at this point?
sudo update-grub
sudo reboot

    At this point everything is working as before. I just don't know why.

    • -1