How To Fix Arch Linux After Windows Breaks It

If you dual boot Windows and Linux, especially if your Windows installation uses BitLocker, you may run into situations where Windows renders your Linux installation unbootable. If you are in a situation where you may need to boot into Windows every once in a while, this guide will show you how to fix your Linux installation after Windows inevitably breaks it.

This issue presents itself on my machine as a failure to load GRUB, as well as some random kernel filesystem errors when loading the boot partition in a Linux chroot environment. My Linux installation is on a completely separate drive from the Windows one, so this behavior is especially puzzling and frustrating.

This guide is mainly for Arch Linux but will likely work with other Linux distros as well with some modifications, but I recommend using the Arch ISO to repair your installation. You should have moderate Linux knowledge before using this guide. Read the guide in its entirety before following it.

Notes

RUN ALL COMMANDS FROM THIS GUIDE AT YOUR OWN RISK. I AM NOT LIABLE FOR ANY DAMAGE OR LOSS OF DATA.
MAKE SURE YOU HAVE A SEPARATE /boot PARTITION. Check via df -h on the Live USB.
This guide assumes you are using an Arch-based distro with GRUB.

Set Up

First, you need a bootable USB drive with Arch on it. From Windows, you can use Rufus to flash it.
Boot into the USB.
Run df -h and locate your Linux partitions.

Reformat Boot Partition

If you are using Arch, feel free to simply wipe your /boot partition without backing it up, as long as you are only storing bootloader and kernel stuff in that partition. Replace /dev/sda1 with your boot partition’s device path.

mkfs.fat -F32 /dev/sda1 # Reformat /boot partition

Mount partitions

mount /dev/sda2 /mnt # Root partition
mount /dev/sda1 /mnt/boot # Boot partition
mount /dev/sda3 /mnt/home # Home partition

Make sure you mount all partitions used by your Linux installation, as we will need the entire filesystem set up for the fstab. If there are others (e.g. separate /home partition), make sure to mount them.

Reinstall GRUB and Linux

# Generate fstab
genfstab -U /mnt > /mnt/etc/fstab

# chroot into existing installation
arch-chroot /mnt

# Reinstall GRUB, again replace /dev/sda with your boot partition's
# device path, not partition path (e.g. NOT /dev/sda1)
grub-install --target=x86_64-efi --efi-directory=/boot /dev/sda

# Reinstall linux (and/or linux-lts), most likely no need for internet since it's
# probably cached in /var/cache/pacman
pacman -S linux linux-lts

# Remake GRUB config
grub-mkconfig -o /boot/grub/grub.cfg

# Clean up
exit
umount -R /mnt
reboot