Microsoft investigating Windows 11's boot errors causing system failures

In the wake of what brought it to a vicious adversity, Microsoft is investigating the reported boot failures in Windows 11 devices following the installation of the latest January 2026 security updates.

The recent Windows 11 error is bringing up "UNMOUNTABLE_BOOT_VOLUME" errors.

What's the bug in Windows 11 that's crashing systems?

The bugs have affected Windows 11 version 25H2 and all editions of version 24H2 after the installation of the KB5074109 cumulative update released as part of this month's updates.

The company is investigating whether this is linked to a Windows update.

In light of complaints, users also reported that systems are failing to start due to a malfunction, showing a stop error during the boot process.

"Microsoft has received a limited number of reports regarding an issue where devices are failing to boot with stop code 'UNMOUNTABLE_BOOT_VOLUME' after installing the January 2026 Windows security update (the originating KBs listed above), released on January 13, 2026, and subsequent updates," the company stated.

Devices impacted by the flaw are displaying a black crash screen with the message: "Your device ran into a problem and needs a restart. You can restart."

The Windows developer indicated that these systems cannot boot into Windows and require manual recovery efforts.

As of now, only physical devices are taking the toll of the bug in question, with no issues noted for virtual machines.

AskWoody, a credible source in the industry, explained that Microsoft is urging affected users and administrators to submit feedback via the Feedback Hub app.

"We are investigating this issue and will confirm if it is a regression caused by a Windows update once we have more details," Microsoft added.

The company has released emergency updates to address a separate issue causing Microsoft Outlook to freeze when PST files are stored in cloud services like OneDrive or Dropbox.