How to Reinstall GRUB With a Ubuntu Live USB After a Windows Update Breaks Dual Boot

Dual boot systems work beautifully until a big Windows update arrives. You install the update, the computer restarts, and Ubuntu has vanished. The machine boots straight into Windows, or shows a Windows recovery screen, with no GRUB menu and no way to choose Ubuntu.

Windows updates do not usually delete Ubuntu. Your Linux system is still sitting on its own partition, untouched. What the update changed is the boot order or the boot loader files in the shared EFI partition, putting Windows first and leaving GRUB out of the startup path.

The fix is to boot from a Ubuntu live USB, enter your installed system safely from there, and reinstall GRUB so the menu returns. This guide covers the simple boot-order fix, the full manual reinstall, and the checks to do afterwards so both systems boot cleanly.

Quick Answer

Create a Ubuntu live USB, boot it in UEFI mode and choose Try Ubuntu, then check the boot order with efibootmgr — often just moving Ubuntu back to first place restores the menu. If not, mount your Ubuntu and EFI partitions, chroot into the installed system, and run grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=ubuntu followed by update-grub. Reboot and the GRUB menu with both Ubuntu and Windows will return.

What a Windows Update Actually Changes

On a modern dual boot computer, both Windows and Ubuntu keep small boot loader files in a shared area called the EFI System Partition. The computer firmware keeps a list, called the boot order, that says which boot loader to run first.

Major Windows updates rewrite their own boot files and frequently set the Windows Boot Manager back to the top of that list. Sometimes they also overwrite settings that GRUB relied on, or a feature update changes the EFI partition contents. The firmware then runs Windows directly and never offers GRUB at all.

This is a boot configuration problem, not a deleted operating system.

Quick Diagnosis Before the Full Repair

1. Restart and open your firmware or one-time boot menu, often with F12, F2, Esc, or Del depending on the brand.
2. If an Ubuntu entry appears in the boot device list, select it. If GRUB and Ubuntu load normally, your GRUB is intact and only the boot order changed.
3. Boot Windows and open Disk Management. Your Ubuntu partitions will appear as healthy partitions without a drive letter. That confirms Ubuntu is still there. Do not format or delete them.
4. Note whether Secure Boot is on and whether your machine uses UEFI. Almost all modern dual boot systems do, and this guide assumes UEFI.

What You Need

• A USB stick of at least 4 GB that you can erase, with a Ubuntu live image written to it using a tool such as Rufus on Windows or Startup Disk Creator on Ubuntu.
• About thirty minutes without rushing, and your charger plugged in if it is a laptop.
• The exact disk layout of your system, which you will identify from the live session rather than guessing.

Writing the stick is covered in the official Ubuntu installation documentation.

Method 1: Fix Only the Boot Order

If the diagnosis showed Ubuntu still boots from the one-time menu, try this five-minute fix first.

1. Boot the live USB, choose Try Ubuntu, open a terminal, and install the boot tool view with sudo apt update && sudo apt install efibootmgr if it is not already present.
2. List the current entries with sudo efibootmgr -v. You will see numbered entries such as Boot0001 for Windows Boot Manager and Boot0002 for ubuntu, plus a BootOrder line.
3. Set Ubuntu first with sudo efibootmgr -o 2,1, using your real numbers in the order you want, Ubuntu first and Windows second.
4. Reboot without the USB and check whether the GRUB menu now appears by default.
5. You can also set the same order permanently in your firmware settings screen, under the boot priority list, which some Windows updates respect more reliably.

If the menu returns, you are done. If it keeps resetting, continue to the full reinstall below. Also disable Fast Startup in Windows power settings.

Method 2: Fully Reinstall GRUB From the Live USB

Step 1: Boot the Live USB in UEFI Mode

1. Insert the USB, restart, and open the one-time boot menu.
2. Choose the entry that begins with UEFI for your USB stick, not the legacy entry. Booting the live session in the wrong mode is the most common reason the later install command fails.
3. Choose Try Ubuntu and wait for the desktop.
4. Confirm the live session itself booted in UEFI mode by running ls /sys/firmware/efi. If files are listed, you are in UEFI mode.

Step 2: Identify and Mount Your Partitions

1. Open a terminal and list the layout with lsblk -f. Find your Ubuntu root partition, usually ext4 and the largest Linux partition, and the small FAT32 EFI partition, usually 100 to 500 MB.
2. Write down both device names. Examples are /dev/nvme0n1p5 for root and /dev/nvme0n1p1 for EFI on an NVMe drive, or /dev/sda5 and /dev/sda1 on a SATA drive.
3. Mount the root partition with sudo mount /dev/sda5 /mnt, using your name.
4. If you have a separate boot partition, mount it at /mnt/boot. Most default Ubuntu installs do not have one.
5. Mount the EFI partition with sudo mount /dev/sda1 /mnt/boot/efi.
6. Bind the live system folders needed inside with for i in /dev /dev/pts /proc /sys /run; do sudo mount --bind $i /mnt$i; done.

Our guide on fixing Ubuntu boot problems that stop at a BusyBox prompt explains disk naming that applies here too.

Step 3: Chroot In and Reinstall GRUB

1. Enter the installed system with sudo chroot /mnt. Your prompt changes because you are now working inside the real Ubuntu installation.
2. Reinstall the boot loader with grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=ubuntu.
3. Wait for a message saying installation finished with no errors. Any error here must be fixed before continuing, usually by checking the EFI mount or the live boot mode.
4. Rebuild the menu with update-grub. Confirm the output lists your Ubuntu kernels and finds the Windows Boot Manager on its partition.
5. Leave the chroot with exit.
6. Unmount everything cleanly with sudo umount -R /mnt, close the terminal, and reboot. Remove the USB when prompted.

Your computer should now show GRUB on every start. Test both entries once.

After the Repair: Checks and Prevention

• Inside Ubuntu, run sudo efibootmgr -v once more and confirm ubuntu is first in the boot order.
• In Windows, keep Fast Startup disabled. It reduces the risk of disk and boot conflicts on dual boot machines.
• Expect a future Windows update may move Windows back to first place. Method 1 is then usually all you need.
• Keep your live USB. It is the universal key for boot repairs on a dual boot system.
• Never use the Windows option to delete or format the unknown partitions. Those are Ubuntu, and Windows simply cannot read their file system.

The community-maintained Ubuntu UEFI documentation page gives useful background on EFI partitions and Secure Boot behaviour for dual boot setups.

Common Mistakes and Useful Tips

• Booting the live USB in legacy mode: The GRUB install then targets the wrong boot style and fails or installs nothing useful. Always pick the UEFI entry and verify with the firmware check.
• Installing GRUB to a partition number: On UEFI systems the install uses the EFI directory method in this guide, not a partition target. Follow the command exactly.
• Mounting the wrong root partition: Dual boot disks can hold recovery and data partitions of similar size. Use file system type and size in lsblk -f to be certain before mounting.
• Forgetting the bind mounts before chroot: Without them, grub-install cannot see devices or EFI variables and produces confusing errors.
• Formatting the EFI partition: Never format it during repair. It is shared by both systems, and Windows needs its files there too. You only mount it and add GRUB back.
• Tip: Take a phone photo of your lsblk -f output before mounting. It makes every later step calmer and gives you an exact record if you ask for help.

Conclusion

When a Windows update breaks dual boot, Ubuntu is almost never gone. The update has simply pushed Windows to the front of the boot order or replaced the shared boot files, leaving GRUB with no turn to run.

Start with the boot-order check, because for many machines that alone restores the menu. If GRUB itself is damaged, boot a live USB in UEFI mode, mount the root and EFI partitions, chroot into your real system, and reinstall GRUB with update-grub to rebuild the menu. Disable Fast Startup, keep the live USB safe, and the next Windows update becomes a minor inconvenience instead of a lost system.

Frequently Asked Questions

Did the Windows update delete my Ubuntu files?

Almost certainly not. Feature updates change boot files and boot order, not your Linux partitions. If Disk Management in Windows shows your Ubuntu partitions as present, your system and files are still there.

Why does my PC boot straight to Windows with no menu at all?

The firmware is following its boot order list, and Windows Boot Manager is now first. Until GRUB runs, no menu can appear. Restoring ubuntu to first place with efibootmgr or your firmware settings fixes this.

Do I need to disable Secure Boot for this repair?

Normally no. Ubuntu’s GRUB is signed for Secure Boot. Keep Secure Boot on and install GRUB using the standard Ubuntu packages as shown in this guide.

What if update-grub does not find Windows?

Make sure the Windows EFI files still exist on the EFI partition and that os-prober is enabled. On recent Ubuntu versions you may need to set GRUB_DISABLE_OS_PROBER=false in /etc/default/grub and run update-grub again.

Can I use Boot-Repair instead of the manual commands?

Yes. The Boot-Repair tool from a live session automates the same mount and reinstall process and suits beginners well. The manual method in this guide is worth learning because it works even when extra tools cannot be installed.