A new issue has been discovered in Windows 11, as a serious bug hidden in the January 2026 security update is causing some PCs to fail to boot properly.
According to Neowin, Microsoft has acknowledged the problem after reports surfaced on the Ask Woody forums. The company stated that a small number of devices experience boot failures after installing the January 13, 2026 Windows security update or later updates.
Affected computers display a black screen with the stop code “UNMOUNTABLE_BOOT_VOLUME.” Users may see a message saying:
“Your device ran into a problem and needs a restart. You can restart.”
However, the system cannot complete the startup process, leaving users unable to access their PC without performing manual recovery steps.
Microsoft says the problem mainly affects Windows 11 versions 24H2 and 25H2, though the company has emphasized that only a limited number of systems appear to be impacted.
For those affected, recovery typically requires using the Windows Recovery Environment (WinRE), which can help repair the operating system and restore the device to a working state.
Analysis: Another frustrating Windows update issue
A key question is how widespread the problem actually is. While Microsoft describes the impact as limited, it has not clarified which editions of Windows 11—such as Home, Pro, or enterprise versions—might be more vulnerable.
Reports on Reddit suggest that several users have experienced boot failures after installing the update. In one discussion thread, multiple users reported similar issues.
One user wrote that their PC completely failed to boot and the recovery process corrupted the boot drive, forcing them to reformat the drive and reinstall Windows. Another user said they encountered a disk read error, and attempts to repair the system through the recovery environment were unsuccessful.
Fortunately, not every case is as severe. Some users reported that performing a system restore successfully fixed the problem and allowed their computers to boot normally again.
At the moment, there are only scattered reports of this issue, but the consequences can be serious for affected systems.
Some users claim that the emergency patch KB5078127 released by Microsoft may resolve the boot failure problem. However, according to reports from Windows Latest, Microsoft has not officially confirmed that this patch addresses the boot issue. The update does fix several other problems, including application crashes in Windows 11, particularly affecting Microsoft Outlook.
For users who have already installed the January update and are not experiencing boot issues, there is likely nothing to worry about. But those who have delayed installing the update due to previous bug reports may want to wait a little longer while Microsoft continues investigating the issue.
Normally, skipping Windows updates is not recommended because they contain important security fixes. However, given the potential severity of this bug—even if it appears to be rare—some users may prefer to wait until a confirmed fix is available.
Meanwhile, there is still no official update regarding another January patch issue that reportedly causes sleep mode to malfunction on some older PCs.
