scripts-fabq/notes/efi-repair.md

EFI Partition Repair

Table of Contents

• EFI Partition Repair
  • Table of Contents
  • Overview
  • Placeholders
  • Repair Procedure
    • 1. Verify What You're Working With
    • 2. Mount Everything for Chroot
    • 3. Chroot In
    • 4. Reinstall GRUB
    • 5. Regenerate GRUB Config
    • 6. Verify fstab
    • 7. Exit and Reboot

Overview

Use this procedure when a Debian machine fails to boot but the root filesystem appears intact — for example after a failed kernel upgrade, a corrupted EFI partition, or GRUB being wiped by another OS. Boot from a live environment, chroot into the installed system, and reinstall GRUB.

Placeholders

Replace the placeholders below with the appropriate values for your setup:

• Devices

  • Disk: <disk> (e.g., /dev/sda, /dev/nvme0n1)
  • EFI partition: <efi-part> (e.g., /dev/sda1, /dev/nvme0n1p1)
  • Root partition: <root-part> (e.g., /dev/sda2, /dev/nvme0n1p2)

• Filesystem

  • Root subvolume name: <root-subvol> (e.g., @rootfs)

Repair Procedure

1. Verify What You're Working With

Identify the disk layout, partition types, UUIDs, and current mount points:

lsblk -o NAME,SIZE,FSTYPE,PARTTYPE,UUID,MOUNTPOINT

Confirm which partition is EFI and which is root before proceeding.

2. Mount Everything for Chroot

Mount the root filesystem

mkdir -p /mnt
mount -o subvol=<root-subvol> <root-part> /mnt

Mount the EFI partition

mkdir -p /mnt/boot/efi
mount <efi-part> /mnt/boot/efi

Bind mount the pseudo-filesystems

mkdir -p /mnt/{dev,proc,sys,run}
mount --bind /dev  /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys  /mnt/sys
mount --bind /run  /mnt/run

Bind mount EFI vars

Required for grub-install to write boot entries to the firmware:

mount --bind /sys/firmware/efi/efivars /mnt/sys/firmware/efi/efivars

3. Chroot In

chroot /mnt /bin/bash
source /etc/profile
export PS1="(chroot) $PS1"

4. Reinstall GRUB

Run grub-install on the disk

grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=debian --recheck <disk>

If you get an error about EFI vars not being writable:

mount -o remount,rw /sys/firmware/efi/efivars

Then re-run grub-install.

5. Regenerate GRUB Config

update-grub

Check the output — it should find your kernel on the btrfs volume. If it complains about not finding a kernel, you may need to also reinstall it:

Check what kernel is installed

dpkg -l | grep linux-image

Reinstall the kernel if missing or broken

apt-get install --reinstall linux-image-amd64

Then re-run update-grub.

6. Verify fstab

Confirm the EFI and root entries look correct before rebooting:

cat /etc/fstab

7. Exit and Reboot

Exit the chroot

exit

Unmount everything

umount -R /mnt

Reboot

reboot