Microsoft is investigating reports that some Windows 11 devices are failing to boot with "UNMOUNTABLE_BOOT_VOLUME" errors after installing the January 2026 Patch Tuesday security updates.

The issue affects Windows 11 version 25H2 and all editions of Windows 11 version 24H2 after installing the KB5074109 cumulative update released as part of this month's updates.

According to Microsoft, some customers have reported that their systems fail to start and display a stop error during boot.

"Microsoft has received a limited number of reports of an issue in which 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 January 13, 2026, and later updates," Microsoft stated.

The company says that affected devices will show a black Windows crash screen stating, "Your device ran into a problem and needs a restart. You can restart."

Microsoft says the reported systems are unable to boot into Windows and require manual recovery efforts to boot again.

Only physical devices are reported as impacted by the bug, with no virtual machines reported so far.

Microsoft is asking affected users and administrators to submit feedback via the Feedback Hub app.

Microsoft says it is investigating whether the behavior is linked to a Windows update.

"We are investigating this issue, and we will confirm if this is a regression caused by a Windows update when we have further details. We will update this documentation when more information is confirmed," Microsoft said.

Microsoft also released emergency out-of-band (OOB) updates on Saturday to fix an issue that caused Microsoft Outlook to freeze when PST files are stored in cloud services such as OneDrive or Dropbox.

The company said this issue primarily affects the Outlook class, which is typically used in enterprise environments.