Microsoft is investigating reports of serious boot failures affecting Windows 11 devices following the January 2026 security updates. Users have reported encountering “UNMOUNTABLE_BOOT_VOLUME” errors, preventing systems from starting and causing widespread disruption.
The bugs impact Windows 11 version 25H2 and all editions of version 24H2 after installing the KB5074109 cumulative update released earlier this month. Affected devices display a black screen with the message that the system ran into a problem and needs a restart.
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According to Microsoft, only physical devices appear to be affected so far, with virtual machines showing no issues. Users experiencing the boot failure are required to perform manual recovery steps to regain access to their systems.
Microsoft confirmed it has received a limited number of reports and is actively investigating whether the problem is linked to the latest security updates. The company urged users and administrators to submit detailed feedback through the Feedback Hub app.
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Industry sources note that the error stops Windows from loading entirely, and administrators are being advised to delay installation of the January 2026 updates until Microsoft provides a fix. Emergency updates have been released separately to address Outlook freezing issues caused by PST files stored in cloud services.
Microsoft stated that it will provide further guidance once investigations confirm the cause and a resolution is ready. The company emphasizes its commitment to resolving the issue quickly to ensure Windows 11 users can safely update without encountering boot failures.