A client recently upgraded their Windows 10 machine to Windows 11.

The "Error loading operating system" message in Windows typically indicates a problem with the boot process. This could be due to corrupted system files, boot configuration data (BCD) errors, or issues with the hard drive.

Windows has a built-in automated startup recovery tool designed to diagnose and fix these types of problems. It attempts to automatically find and repair issues that are preventing Windows from starting correctly.

Here's why the other options are not the best first step:

A. Disable the Secure Boot feature: Secure Boot is a security feature that helps prevent malware from loading during startup. While it can sometimes cause issues with incompatible hardware or software, it's unlikely to be the primary cause of this error after a Windows upgrade.

C. Format the drive: Formatting the drive should be a last resort as it will erase all data on the hard drive.

D. Resize the boot partition: Resizing the boot partition is a complex task and not usually necessary to fix a boot error.

Troubleshooting Steps:

Use the automated startup recovery:

This is usually attempted automatically when Windows fails to boot.

If it doesn't start automatically, you might need to access the Advanced Startup Options menu (usually by repeatedly pressing F8 or Shift+F8 during startup).

If automated repair fails, try other options from the Advanced Startup Options menu:

System Restore: Restore the system to a point before the upgrade.

Startup Repair: This tool can fix more complex boot errors.

Command Prompt: Use the command prompt to run commands like bootrec /fixmbr, bootrec /fixboot, and bootrec /rebuildbcd to repair the boot configuration.