Reports indicate that certain Windows 11 devices are encountering boot failures, specifically displaying ‘UNMOUNTABLE_BOOT_VOLUME’ errors, following the installation of the January 2026 Patch Tuesday security updates. Microsoft is currently investigating these incidents.
This problem impacts Windows 11 version 25H2 and all editions of Windows 11 version 24H2. It occurs after the installation of the KB5074109 cumulative update, which was part of January’s updates.
Microsoft has acknowledged that some users have reported their systems failing to start, presenting a stop error during the boot process.
A limited number of reports have reached Microsoft concerning devices failing to boot, exhibiting the stop code ‘UNMOUNTABLE_BOOT_VOLUME’. These failures occur after installing the January 2026 Windows security update, released on January 13, 2026, and subsequent updates.
Affected devices reportedly display a black Windows crash screen with the message: ‘Your device ran into a problem and needs a restart. You can restart.’
The affected systems are unable to boot into Windows, necessitating manual recovery procedures to restore functionality.
The bug has only been reported on physical devices; no virtual machines have been affected to date.
Initially highlighted by AskWoody, Microsoft is now encouraging affected users and administrators to submit feedback through the Feedback Hub app. The company is investigating if this behavior is indeed connected to a Windows update.
Microsoft stated, “This issue is under investigation, and confirmation will be provided regarding whether it is a regression caused by a Windows update once further details are available. This documentation will be updated as more information is confirmed.”
Separately, Microsoft also issued emergency out-of-band (OOB) updates on Saturday. These updates addressed a problem causing Microsoft Outlook to freeze when PST files were stored in cloud services like OneDrive or Dropbox.
This particular Outlook issue primarily impacts the Outlook class, commonly utilized in enterprise settings.
