BIOS/MBR to UEFI/GPT with Systemd-boot

Converting NixOS from BIOS/MBR to UEFI/GPT with systemd-boot

Problem Summary

System was using Legacy BIOS boot with MBR partition table
Wanted to switch to UEFI boot with systemd-boot
systemd-boot requires GPT partition table and UEFI mode

Prerequisites

Backup important data
NixOS Live USB for recovery
Access to motherboard BIOS/UEFI settings

Process

1. Create EFI Partition

If no EFI partition exists, create one:

Boot from Live USB
Use GParted to shrink root partition by ~1GB
Create new FAT32 partition
Set boot and esp flags

2. Convert MBR to GPT

sudo gdisk /dev/nvme1n1
# When prompted about MBR vs corrupt GPT, choose: 2 (MBR)
# Then:
r    # Recovery menu
f    # Convert MBR to GPT (preserves partitions)
p    # Preview - verify partitions are correct
w    # Write changes
Y    # Confirm

3. Enable UEFI Boot

Reboot and enter motherboard BIOS (F2/Del/F12)
Change from Legacy/BIOS to UEFI mode
Disable CSM (Compatibility Support Module)
Save and exit

4. Install systemd-boot

Boot from Live USB if needed:

# Mount system
sudo mount /dev/nvme1n1p1 /mnt
sudo mount /dev/nvme1n1p3 /mnt/boot

# Enter chroot
sudo nixos-enter --root /mnt

# Install bootloader
bootctl --path=/boot install

# Generate boot entries
/nix/var/nix/profiles/system/bin/switch-to-configuration boot

# Verify entries exist
ls -la /boot/loader/entries/

5. Get Device UUID for Configuration

Find the UUID of your boot partition:

# Method 1: Using blkid
sudo blkid /dev/nvme1n1p3
# Output: /dev/nvme1n1p3: UUID="XXXX-XXXX" TYPE="vfat"

# Method 2: List all UUIDs
ls -la /dev/disk/by-uuid/ | grep nvme1n1p3

# Method 3: Using lsblk
lsblk -f | grep nvme1n1p3

6. Update NixOS Configuration

Ensure /etc/nixos/configuration.nix has:

boot.loader.systemd-boot.enable = true;
boot.loader.efi.canTouchEfiVariables = true;

Update /etc/nixos/hardware-configuration.nix with the UUID from step 5:

fileSystems."/boot" = {
  device = "/dev/disk/by-uuid/XXXX-XXXX";  # Replace with your UUID
  fsType = "vfat";
  options = [ "umask=0077" ];
};

Note: Using UUID is preferred over /dev/nvme1n1p3 because:

UUIDs persist across reboots
UUIDs don't change if drive order changes
More reliable for boot partitions

Common Issues

"efiSysMountPoint = '/boot' is not mounted"

Mount the EFI partition: sudo mount /dev/nvme1n1p3 /boot
Ensure it's in hardware-configuration.nix with correct UUID

"Found valid MBR and corrupt GPT"

Choose option 2 (MBR) to preserve existing partitions

Use activation script: /nix/var/nix/profiles/system/bin/switch-to-configuration boot

Stuck at GRUB prompt after conversion

Boot from Live USB and install systemd-boot as shown above

Verification

Check boot mode: [ -d /sys/firmware/efi ] && echo "UEFI" || echo "BIOS"
Check partition table: sudo parted /dev/nvme1n1 print | grep "Partition Table"
Should show "gpt" and boot in UEFI mode