- There’s a new bug in Windows 11’s January Update
- Microsoft informs us that this causes startup errors based on a “limited number of reports”
- If it is affected, please try a manual restore and Microsoft is now investigating this issue
Windows 11 has another serious bug hidden in the January update, and this one is a showstopper that means affected PCs can’t boot.
Neowin reports that Microsoft has acknowledged the bug with a message as flagged via the Ask Woody forums: “Microsoft has received a limited number of reports of an issue where devices fail to boot with the stop code ‘UNMOUNTABLE_BOOT_VOLUME’ after installing the January 2026 Windows Security Update, released on January 1, 2026, and later updates.”
“Affected devices show a black screen with the message ‘Your device ran into a problem and needs to restart. You can restart’. At this stage, the device cannot complete booting 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 affected by the bug according to Microsoft. The company said the issues relate 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, which means using the Windows Recovery Environment (WinRE). It can be used to attempt to repair the system and return it to a functional state.
Analysis: more error misery – and an obvious question
The question in this case is: how ‘limited’ is limited? This suggests that a small spread of people on Windows 11 25H2 and 24H2 are affected, although Microsoft does not elaborate on which variants of the operating system may be at risk 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 startup errors caused by this latest Windows update.
One Redditor notes: “Can confirm. My PC didn’t actually boot. Recovery completely corrupted the boot drive and needed a full reformat and reinstall.” Another said they got a “disk read error” and a repair operation (via the recovery environment) didn’t work to fix this. A further report says that a system restore fixed their PC when they couldn’t boot, thankfully, so not everyone here runs into a brick wall trying to restore their machine. (Note that these generally appear to be consumer PCs – as opposed to work computers).
Now, I don’t see any other reports of this, but this update has definitely 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’s not part of the claimed fixes here, according to what Windows Latest has heard from Microsoft. However, that emergency patch fixes various gremlins that cause apps to crash in Windows 11, including Outlook – and it’s obviously worth using anyway. Even if you’ve already installed the January update and haven’t run into startup errors, you’re still out.
However, the people who have put off the January update due to the various reported bugs may want to stay away for a while longer until Microsoft investigates this new issue further (a process that is ongoing according to Neowin).
I wouldn’t normally recommend not installing a Windows 11 update – due to not having the security patches in it – but in this case, as one Redditor put it, the damage to their PC from the update was worse than any virus. (Not likely, actually – but you get the gist). Obviously, that’s your decision to make, and it looks like the bug is rare – but that doesn’t mean it’s impossible for it to hit if you install the January update.
Still no news on the January update bug that allegedly torpedoes hibernation on some (older) PCs.
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