Key Takeaways
1. Boot Failure Issue: The January 2026 Windows 11 security update KB5074109 has caused some computers to experience UNMOUNTABLE_BOOT_VOLUME errors, leading to complete boot failures.
2. Limited Reports: The problem appears to affect a small percentage of devices running Windows 11 versions 24H2 and 25H2, as noted in community reports.
3. Microsoft Investigation: Microsoft is investigating these boot issues but has not identified the exact cause or provided a hotfix yet.
4. Recovery Recommendations: Users facing boot problems are advised to use the Windows Recovery Environment (WinRE) or external media to repair or revert their systems.
5. Caution Advised: Users whose systems are functioning normally should remain calm, but system administrators may be more cautious with updates until a reliable solution is provided by Microsoft.
Microsoft’s January 2026 Windows 11 security update KB5074109 has been associated with a significant problem beyond just crashes and application malfunctions: some computers are experiencing complete boot failures, displaying a UNMOUNTABLE_BOOT_VOLUME blue screen.
Limited Reports of Boot Failures
As noted in Microsoft’s internal status documents, which have been compiled by community sources like AskWoody, along with recent findings from BleepingComputer and Windows Central, there are “limited number of reports” indicating that Windows 11 devices running on version 24H2 and 25H2 encounter a UNMOUNTABLE_BOOT_VOLUME error after applying the January 13 security updates related to KB5074109.
In these instances, the computers do not reach the login page, instead getting stuck in a loop that leads to a BSOD. This situation leaves users needing to recover their systems through the Windows Recovery Environment (WinRE) or by using external media.
Ongoing Investigation by Microsoft
Microsoft is currently looking into these boot issues and has stated that they appear to only impact a small percentage of devices. However, the company has not yet identified the precise cause of the UNMOUNTABLE_BOOT_VOLUME problem nor released a specific hotfix. Their public health pages primarily address issues like Remote Desktop failures, black screens, and crashes of cloud applications that are linked to KB5074109 and the January out-of-band updates.
For now, Microsoft recommends that users facing the UNMOUNTABLE_BOOT_VOLUME issue follow the same steps as for other boot problems: utilize WinRE and standard recovery methods to either repair or revert the system. This involves booting from either the recovery environment or installation media, trying to repair the file system, and, when feasible, uninstalling the January security update before Windows starts.
No Need for Alarm
For those whose computers are still functioning properly after the KB5074109 update, there’s no reason to panic. However, this new stop-code problem highlights the vulnerability of the January Windows 11 update process. With emergency hotfixes, rollback issues, and now complete boot failures, many system administrators will approach KB5074109 with heightened caution until Microsoft provides a fully reliable cumulative update.
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