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Microsoft’s January 2026 security update for Windows 11 is still causing problems. The patch initially introduced issues that prevented select computers from shutting down or going to sleep, followed by problems with Outlook and Remote Desktop. Microsoft now confirms it has received a "limited number of reports” of users unable to boot their computers at all.
As shared by a user on the Ask Woody forum and later spotted by Bleeping Computer, some users are seeing the message “UNMOUNTABLE_BOOT_VOLUME” on their computers after installing the KB5074109 update. All affected devices are running Windows 11 25H2 or 24H2.
It shows a black screen with the message, “Your device ran into a problem and needs a restart. You can restart.” There’s currently no way around this other than what Microsoft calls "manual recovery steps," which likely means reverting to a previous version of Windows 11.
We don't know how many people are affected, or why these issues affect some devices, but not others. “Reports received so far indicate that this issue is limited to physical devices only; no customers have reported observing these symptoms on virtual machines," Microsoft says.
The company confirms that it's investigating the problem, but hasn't shared details about a patch. If it follows a similar routine to its last update, we may see a fix in the coming days.
If you have the issue and you don't want to wait for more information from Microsoft, you could try reverting to an older version of the software yourself. To do so, check out PCMag's guide on how to install a previous version of Windows.
This tumultuous security update was first released on Jan. 13, followed by further tweaks (KB5077797 and KB5078127) to address the original issues.