Repairing the NixOS Bootloader after BIOS/Windows Updates

I’ve been using NixOS as my main OS for nearly 3 years now, but I still dual-boot Windows for work and some software compatibility. After a Windows update overwrote my Linux bootloader and a recent BIOS update did it again, I decided to write down the steps I took to fix the issue for future reference. Your mileage may vary depending on the specifics of what happened, but here are the general steps.

Creating a Live USB

The first step is to create a NixOS live USB to boot into an environment with the correct tools. You can download the latest ISO image from the NixOS website. I selected the “Minimal” image since it’s smaller (~1.5 GB). Use a tool like Ether or USBImager to write the ISO to a USB stick.

Booting from the Live USB

Insert the USB stick into your computer and boot from it. You may need to change the boot order in your BIOS/UEFI settings. On my laptop, you get there by spamming F2 during startup.

Once you launch the installer, you should be dropped into a terminal.

Check partition layout

Identify your main disk under the /dev directory. Then check the partition layout to make sure things look correct:

sudo fdisk -l /dev/nvme0n1   # or /dev/sda, etc.

Identify the NixOS root partition and the EFI boot partition. It’ll probably be something like /dev/nvme0n1p3 for root and /dev/nvme0n1p1 for EFI, but double check to be sure.

Mount partitions

If you have full-disk encryption with LUKS, unlock the encrypted partition first:

sudo cryptsetup luksOpen /dev/<whatever-partition> tmp-root

Then mount the NixOS root partition:

sudo mount /dev/mapper/tmp-root /mnt

Mount the EFI partition:

sudo mount /dev/<whatever-efi-partition> /mnt/boot

Enter the system and reinstall the bootloader

See the NixOS wiki for more info.

nixos-enter
NIXOS_INSTALL_BOOTLOADER=1 /nix/var/nix/profiles/system/bin/switch-to-configuration boot

Reboot

Hopefully that worked and you can now reboot/modify boot order if necessary/profit!

Troubleshooting

If for whatever reason the partitions don’t look correct or cannot be mounted, you may need to try some repair steps. This forum post has some info.