- There’s a new bug in Windows 11’s January update
- Microsoft informs us that this one is causing boot failures based on a “limited number of reports”
- If affected, you’ll have to try a manual recovery, and Microsoft is now investigating this issue
Windows 11 has another serious bug hidden in the January update, and this is a showstopper that means affected PCs fail to boot up.
Neowin reports that Microsoft has acknowledged the bug with a message as flagged up via the Ask Woody forums: “Microsoft has received a limited number of reports of an issue in which devices are failing to boot with stop code ‘UNMOUNTABLE_BOOT_VOLUME’, after installing the January 2026 Windows security update, released January 13, 2026, and later updates.
“Affected devices show a black screen with the message ‘Your device ran into a problem and needs a restart. You can restart.’ At this stage, the device cannot complete startup and requires manual recovery steps.”
So, the good news is that we’re told there’s a limited impact here, so not many PCs are hit by the bug according to Microsoft. The company said that the issues pertain to Windows 11 versions 24H2 and 25H2.
The not-so-great news is that it’s a nasty bug, and as Microsoft notes, you’ll need to go through a manual recovery, meaning using the Windows Recovery Environment (WinRE). That can be used to try and repair the system, returning it to a functional state.
Analysis: more bug misery – and an obvious question
The question in this case is: how ‘limited’ is limited? This would suggest a small scattering of people on Windows 11 25H2 and 24H2 are hit, though Microsoft doesn’t elaborate on what flavors of the OS might be in danger here (in terms of Windows 11 Home or Pro, or enterprise editions).
Looking on Reddit, which is the most likely place to find (relatively reliable) reports of problems with Windows updates, I came across this thread with seven separate complaints about boot failures caused by this latest Windows update.
One Redditor notes: “Can confirm. My PC did in fact not boot. Recovery fully corrupted the boot drive and needed a full reformat and reinstall.” Another said they were getting a “disk read error” and that a repair operation (via the recovery environment) didn’t work to resolve this. A further report says that a system restore did fix their PC failing to boot, thankfully, so not everyone here is running into a brick wall trying to recover their machine. (Note that these all appear to be consumer PCs in the main – as opposed to work computers).
Now, I can’t see any other reports of this, but this update has certainly ended badly for some folks, based on the (anecdotal) complaints that are floating around. Some also claim that the new emergency patch issued by Microsoft – KB5078127 – cures this boot failure issue, but that isn’t part of the claimed resolutions here going by what Windows Latest has heard from Microsoft. That emergency patch does fix various gremlins causing apps to crash in Windows 11, including Outlook, though – and it’s obviously worth applying anyway. Although if you’ve already installed the January update, and haven’t hit boot failures, then you’re in the clear anyway.
Those people who’ve held off the January update due to the various reported bugs, however, may want to stay away for a while longer until Microsoft further investigates this fresh problem (a process that’s underway according to Neowin).
I wouldn’t normally recommend not installing a Windows 11 update – due to being without the security fixes applied therein – but in this case, as one Redditor put it, the damage done to their PC by the update was worse than any virus. (Not likely, in fact – but you get the gist). Obviously, it’s your decision to make, and it does seem like the bug is rare – but that doesn’t mean it’s impossible that it’ll strike if you install the January update.
There’s still no news on the January update glitch that’s reportedly torpedoing sleep mode on some (older) PCs.
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