KB5074109 Update Causes Boot Failures on Windows 11 Devices

Microsoft has struggled with update-related issues since releasing the January 2026 Patch Tuesday updates for Windows 11. While the company already pushed emergency fixes for shutdown problems and Azure Virtual Desktop disruptions, new reports suggest the situation is not fully resolved.

According to BleepingComputer, Microsoft is now investigating reports of some Windows 11 devices failing to boot after installing the latest security updates.

Windows 11 PCs hit by UNMOUNTABLE_BOOT_VOLUME error

Affected systems show a black crash screen with the error message “UNMOUNTABLE_BOOT_VOLUME” during startup. In many cases, users must rely on manual recovery steps to get the system running again.

So far, reports only mention physical devices. Virtual machines do not appear affected at this time.

The issue appears linked to the KB5074109 cumulative update, which Microsoft released in mid-January as part of Patch Tuesday. Users began reporting boot failures shortly after installing the update.

Microsoft has acknowledged the reports and is actively investigating whether KB5074109 caused the regression. The company has asked affected users and IT administrators to submit logs and feedback through the Feedback Hub to help identify the root cause.

This boot failure bug adds to a growing list of Windows 11 problems tied to the January 2026 updates. Users have also reported reboot loops, system instability, and other post-update issues related to KB5074109.

Alongside the boot problems, many users reported that Outlook Classic became unusable after installing the January updates. Microsoft has already released an emergency update that resolves the Outlook issue, but other Windows 11 bugs remain under investigation.

Microsoft has not shared a timeline for a fix yet. Users experiencing boot failures may want to delay installing the affected update until more information becomes available.