Microsoft is looking into reports of some Windows 11 devices encountering "UNMOUNTABLE_BOOT_VOLUME" errors and failing to boot after the January 2026 Patch Tuesday security updates. This problem impacts Windows 11 version 25H2 and all editions of Windows 11 version 24H2 following the installation of the KB5074109 cumulative update included in this month's updates. Microsoft has noted that some customers have reported their systems not starting and showing a stop error during boot. Affected devices will display a black Windows crash screen with the message, "Your device ran into a problem and needs a restart. You can restart." Microsoft reports that the affected systems cannot boot into Windows and need manual recovery to restart. Only physical devices are impacted by the bug, with no virtual machines affected so far. Microsoft also released emergency out-of-band (OOB) updates on Saturday to resolve an issue that caused Microsoft Outlook to freeze when PST files are stored in cloud services like OneDrive or Dropbox.