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GRUB: Manually Adding Windows to Ubuntu Boot Menu

When you need this: Ubuntu doesn’t detect Windows in GRUB after install, or update-grub finds it but then loses it after kernel updates. This guide covers manual entry via 40_custom - the permanent, update-proof approach.

Warning: Editing GRUB incorrectly can prevent the system from booting. Read each step before running it. Have a live USB ready as a fallback.

Prerequisites - Understand Your Setup First

Is your system UEFI or Legacy BIOS?

[ -d /sys/firmware/efi ] && echo "UEFI" || echo "Legacy BIOS"

This guide covers UEFI (the common case for any machine made after ~2012). Legacy BIOS uses different chainloader syntax.

Find the Windows EFI partition

sudo fdisk -l

Look for a partition with type EFI System - typically small (100–500 MB), formatted as FAT32. Example output:

Device           Start      End  Sectors  Size Type
/dev/nvme0n1p1    2048   206847   204800  100M EFI System   ← this one
/dev/nvme0n1p2  206848  ..       ...      16M  Microsoft reserved
/dev/nvme0n1p3  ...              ...     ~C:  Microsoft basic data

Note the device path (e.g., /dev/nvme0n1p1 for NVMe, /dev/sda1 for SATA).

Step 1 - Get the EFI Partition UUID

sudo blkid /dev/nvme0n1p1

Example output:

/dev/nvme0n1p1: UUID="2B11-51E9" BLOCK_SIZE="512" TYPE="vfat" PARTLABEL="EFI System Partition"

Copy the UUID value - you’ll use it in the GRUB entry. The short format (e.g., 2B11-51E9) is correct for FAT32 EFI partitions.

Step 2 - Verify the Windows Boot File Exists

Mount the EFI partition and confirm the Windows bootloader is there:

sudo mkdir -p /mnt/efi
sudo mount /dev/nvme0n1p1 /mnt/efi
ls /mnt/efi/EFI/
# Should show: Microsoft  ubuntu  (and possibly other entries)

ls /mnt/efi/EFI/Microsoft/Boot/
# Should include: bootmgfw.efi

sudo umount /mnt/efi

If bootmgfw.efi is missing, Windows boot files are damaged - this GRUB entry won’t fix that. You’d need Windows recovery media.

Step 3 - Add the Custom GRUB Entry

sudo nano /etc/grub.d/40_custom

Add below the existing comment lines (don’t delete the exec tail line at the top):

menuentry 'Windows 11' {
    insmod part_gpt
    insmod fat
    insmod chain
    search --fs-uuid --no-floppy --set=root 2B11-51E9
    chainloader (${root})/EFI/Microsoft/Boot/bootmgfw.efi
}

Why 40_custom and not 40_custom alternatives? Files in /etc/grub.d/ are executed in numeric order. 40_custom runs after OS detection scripts (10_linux, 30_os-prober) but before the closing script (90_disable_os_prober). Entries here survive update-grub calls and kernel upgrades intact.

Save: Ctrl+O → Enter → Ctrl+X

Step 4 - Set Permissions

sudo chmod o-w /etc/grub.d/40_custom
# Remove write permission for others - GRUB ignores world-writable scripts as a security measure

Step 5 - Update GRUB and Reboot

sudo update-grub

Check the output - you should see a line like:

Found Windows Boot Manager on /dev/nvme0n1p1@/EFI/Microsoft/Boot/bootmgfw.efi

Or for the custom entry:

Adding boot menu entry for GRUB bootloader

sudo reboot

Windows 11 should now appear in the GRUB menu.

GRUB Customization (Optional)

Change default boot entry

sudo nano /etc/default/grub

# Boot the first entry (Ubuntu) by default - 0-indexed
GRUB_DEFAULT=0

# Or boot a specific entry by name
GRUB_DEFAULT="Windows 11"

# Or remember the last selection
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true

sudo update-grub

Change boot menu timeout

# In /etc/default/grub
GRUB_TIMEOUT=10      # show menu for 10 seconds
GRUB_TIMEOUT=0       # boot immediately (no menu shown)
GRUB_TIMEOUT=-1      # wait indefinitely for user input

One-time boot override (no config change needed)

# Boot into Windows on the next reboot only, then revert to default
sudo grub-reboot "Windows 11"
sudo reboot

# Set a new permanent default by entry index
sudo grub-set-default 0

To find entry indices:

grep -E "^menuentry|submenu" /boot/grub/grub.cfg | nl -ba

Troubleshooting

update-grub doesn’t find Windows automatically

os-prober may be disabled. Check:

grep GRUB_DISABLE_OS_PROBER /etc/default/grub

If it’s set to true, either remove the line or set it to false, then run sudo update-grub. Note: on some distros os-prober is disabled by default since GRUB 2.06.

GRUB menu doesn’t appear at all (boots straight to Ubuntu)

# In /etc/default/grub, ensure:
GRUB_TIMEOUT_STYLE=menu   # not "hidden" or "countdown"
GRUB_TIMEOUT=5

Then sudo update-grub.

Entry appears but selecting Windows gives error

Common errors and causes:

Error Cause
error: file '/EFI/Microsoft/Boot/bootmgfw.efi' not found Wrong UUID, or EFI partition not mounted as root
error: invalid signature Secure Boot mismatch - try disabling Secure Boot in BIOS
Boots to Windows recovery Windows fast startup wrote dirty state - disable Fast Startup in Windows

Verify UUID is correct:

sudo blkid | grep -i efi

Check Windows Fast Startup (in Windows): Control Panel → Power Options → Choose what the power buttons do → Turn off fast startup.

Revert a bad GRUB change

If the system won’t boot after editing GRUB, boot from Ubuntu live USB and:

sudo mount /dev/nvme0n1p2 /mnt          # mount your Ubuntu root partition
sudo mount /dev/nvme0n1p1 /mnt/boot/efi # mount EFI
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo chroot /mnt

# Fix the bad config
nano /etc/grub.d/40_custom
update-grub
exit
sudo reboot

Notes

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