Fix Dual-Boot Issues With Windows on Ubuntu

After a Windows update, a dual-boot system with Ubuntu may suddenly boot straight into Windows, skip the GRUB menu, or fail to start Linux altogether. These problems often stem from changes in bootloader configuration, Secure Boot settings, or mismatched system clocks. Addressing these issues quickly restores access to both operating systems without data loss or a full reinstall.

Windows updates, especially major version upgrades, can overwrite the bootloader settings, causing the system to bypass the GRUB menu and boot directly into Windows. To restore the dual-boot menu:

Step 1: Open Windows and launch Command Prompt as an administrator. Right-click the Start menu, select Command Prompt (Admin) or Windows Terminal (Admin).

Step 2: Enter the following command to set the boot manager to GRUB. This command tells Windows to use the Ubuntu bootloader at startup:

bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi

This step ensures that GRUB appears on reboot, allowing you to select either Ubuntu or Windows. If your system uses shimx64.efi (common on Secure Boot systems), adjust the path accordingly:

bcdedit /set {bootmgr} path \EFI\ubuntu\shimx64.efi

Step 3: Restart your computer. The GRUB menu should now display both operating systems.

Repairing Bootloaders When GRUB or Windows Fails to Boot

Sometimes, the system may not recognize either Ubuntu or Windows after a bootloader repair or update. If GRUB fails to load or selecting Windows in GRUB produces errors such as "cannot load image":

Step 1: Use a live Ubuntu USB stick to boot into a live session. Connect to the internet.

Step 2: Open a terminal and install Boot Repair:


sudo add-apt-repository -y ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair

Step 3: Launch Boot Repair from the applications menu. Choose the recommended repair option. Follow any instructions provided by the tool, such as running highlighted commands in the terminal. This process reinstalls GRUB and updates boot entries.

Step 4: Once Boot Repair completes, reboot and check if both Ubuntu and Windows are accessible from the GRUB menu.

Fixing Windows Boot Issues After GRUB Repairs

On rare occasions, repairing GRUB or running update-grub can corrupt Windows boot files, resulting in errors when attempting to start Windows from GRUB or UEFI. If Windows fails to boot even when selected directly from the UEFI boot menu, a deeper repair is required:

Step 1: Download the official Windows installation media creation tool from Microsoft’s website. Use it to create a Windows recovery USB drive.

Step 2: Boot from the USB drive. In the setup menu, select your language, then click the “Repair” or “Troubleshoot” option instead of proceeding with installation.

Step 3: Open the Command Prompt from the recovery options.

Step 4: Use diskpart to identify and assign drive letters to your EFI (boot) and Windows partitions. For example:


diskpart
list disk
select disk #
list partition
select partition #   (choose EFI partition, usually 300-500MB)
assign letter=n:
select partition #   (choose Windows partition)
assign letter=c:
exit

Step 5: Update the boot sector and rebuild boot configuration data:


bootsect /nt60 all /force
c:
bcdboot c:\windows /s n: /f all
bootrec /scanos
bootrec /fixmbr
bootrec /fixboot   (skip if “access denied” appears)
bootrec /rebuildbcd

Step 6: Exit and reboot. Windows should now boot correctly. To restore the GRUB menu as the default, access the UEFI settings and move the Ubuntu entry to the top of the boot order.

Resolving Secure Boot and SBAT Security Policy Errors

Recent Windows updates may activate Secure Boot Advanced Targeting (SBAT), which blocks older Linux bootloaders and prevents Ubuntu from starting. If you see errors like “Verifying shim SBAT data failed: Security Policy Violation,” follow these steps:

Step 1: Enter your system’s BIOS or UEFI firmware settings by pressing F2, Delete, or the designated key during startup.

Step 2: Disable Secure Boot. Save and exit the firmware settings.

Step 3: Boot into Ubuntu. Open a terminal and run:

sudo mokutil --set-sbat-policy delete

This command removes the problematic SBAT revocation, allowing Ubuntu to boot.

Step 4: Restart and verify that Linux boots. To check SBAT status, use:

mokutil --list-sbat-revocations

If the list is empty, the SBAT block is cleared.

Step 5: Optionally, re-enable Secure Boot in your UEFI settings once Ubuntu is working again.

Correcting Time Differences Between Windows and Ubuntu

Dual-boot setups often display incorrect time in one operating system due to different interpretations of the hardware clock. Windows treats the hardware clock as local time, while Linux assumes it is set to UTC. This mismatch causes a time drift after switching between systems.

Step 1: To make Ubuntu use local time (matching Windows’ expectation), open a terminal in Ubuntu and run:

sudo timedatectl set-local-rtc 1

This command configures Ubuntu to read and write the hardware clock as local time, preventing time discrepancies when switching between OSes.

Step 2: Alternatively, enable automatic time synchronization in both Windows and Ubuntu. In Ubuntu, go to System Settings > Date & Time and enable automatic time and timezone updates. In Windows, enable “Set time automatically” in the Date & Time settings.

If the GRUB menu defaults to Ubuntu and you want Windows to appear first, or if Windows is missing from the GRUB menu:

Step 1: Install GRUB Customizer in Ubuntu by running:

sudo apt install grub-customizer

Step 2: Launch GRUB Customizer, move the Windows entry above Ubuntu using the arrow buttons, and save the changes. This sets Windows as the default boot option.

If Windows is missing from the GRUB menu after installing Ubuntu 22.04 or later, edit /etc/default/grub and add:

GRUB_DISABLE_OS_PROBER=false

Then update GRUB:

sudo update-grub

Applying these targeted fixes restores full dual-boot functionality, resolves time issues, and ensures both Windows and Ubuntu remain accessible after updates or configuration changes.