- There is a new bug in the Windows 11 January Update
- Microsoft tells us that this is causing boot failures based on a “limited number of reports”
- If you are affected, you will need to attempt a manual recovery and Microsoft is now investigating this issue.
Windows 11 has another serious bug hidden in the January update, and this is an issue that means affected PCs won’t start.
Neowin reports that Microsoft has acknowledged the bug with a message flagged via the Ask Woody forums: “Microsoft has received a limited number of reports of an issue where devices do not boot with the stop code ‘UNMOUNTABLE_BOOT_VOLUME’, after installing the January 2026 Windows Security Update, released on January 13, 2026, and subsequent updates.
“Affected devices display a black screen with the message ‘Your device has had a problem and needs to restart. You can restart’. At this stage, the device is unable to complete startup and requires manual recovery steps.”
So the good news is that we’re told there’s limited impact here, so according to Microsoft, not many PCs are affected by the bug. The company said the issues pertain to versions 24H2 and 25H2 of Windows 11.
The not-so-good news is that this is a nasty error and, as Microsoft points out, you’ll need to perform a manual recovery, i.e. use the Windows Recovery Environment (WinRE). This can be used to attempt to repair the system and return it to a functional state.
Analysis: More bug issues and an obvious question
The question in this case is: how “limited” is limited? This would suggest that a small scatter of people on Windows 11 25H2 and 24H2 are affected, although Microsoft doesn’t elaborate on which versions of the operating system could be at risk here (in terms of Windows 11 Home or Pro, or enterprise editions).
Searching Reddit, which is the most likely place to find (relatively reliable) reports of problems with Windows updates, I found this thread with seven separate complaints about boot failures caused by this latest Windows update.
One Redditor notes: “I can confirm. In fact, my PC did not boot. The recovery completely corrupted the boot drive and required a complete reformat and reinstall.” Another said he was getting a “disk read error” and that a repair operation (via the recovery environment) didn’t work to resolve it. Another report says that a system restore fixed your PC’s boot issue, fortunately, so not everyone here is running into a brick wall when trying to recover their machine. (Note that these all appear to be mostly consumer PCs, as opposed to work computers.)
Now, I can’t see any other reports on this, but this update certainly ended badly for some people, based on the (anecdotal) complaints floating around. Some also claim that the new emergency patch issued by Microsoft, KB5078127, fixes this boot failure issue, but that is not part of the resolutions claimed here based on what Windows Latest has heard from Microsoft. However, that emergency patch fixes several gremlins that cause apps to crash in Windows 11, including Outlook, and is obviously worth applying anyway. Although if you’ve already installed the January update and haven’t had boot failures, then you’re safe anyway.
However, those people who have been putting off the January update due to the various reported bugs will want to stay away for a while longer until Microsoft further investigates this new issue (a process that is underway according to Neowin).
Normally I wouldn’t recommend not installing a Windows 11 update, because it doesn’t have the security fixes applied to it, but in this case, as one Redditor said, the damage caused to your PC by the update was worse than any virus. (Actually, it’s not likely, but you get the gist.) Obviously, it’s your call and it seems like the error is rare, but that doesn’t mean it’s impossible for it to happen if you install the January update.
There is still no news on the January update bug that is reportedly torpedoing sleep mode on some (older) PCs.
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