Microsoft reports 'an issue in which devices are failing to boot' after downloading the latest update

If you have downloaded January's KB5074109 Windows update and can't seem to get past boot, you aren't alone.

As reported by Windows Latest, Microsoft has updated users in a support document that it is "investigating customer reports of device boot failures following the January 2026 update and later updates."

The document is only accessible by Microsoft 365 business accounts, which does seem to imply that it's business devices that are typically affected by this boot problem. However, so far, there's been no confirmation either way on which devices are affected.

The document says, "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 (KB5074109), 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."

Typically, this error code tends to signify a problem with a boot drive (but can also happen due to corrupted files or even an issue with memory). However, this isn't the first time users have reported a Windows update being responsible for this error code.

This is not the only problem that has popped up in this month's updates. January 24 saw the launch of a surprise update, which patched an issue with apps becoming unresponsive when saving files on the cloud.

Microsoft has only indicated a 'limited' number of reports, so we don't yet know how widespread this problem is. Microsoft does clarify that it is only currently happening to physical devices and hasn't been spotted on virtual machines. The issue is being investigated and seems to only impact those with the 26200.7623 and 26100.7623 Windows builds, or later.

You can check if your PC could be affected by going to 'Settings', then 'About' and scrolling down to 'Windows info'. The 'OS build' section should tell you which build you're on. Naturally, if you aren't yet on the KB5074109 Windows update, I'd advise holding off until Microsoft can diagnose the problem.

If, however, you have the update and have found yourself affected, you will have to use the Windows Recovery option or, worst-case scenario, go through a clean reinstall. The only thing worse than using Windows is installing Windows, but for casual users, there aren't many OSes to choose from. I've been hearing Linux calling my name like the Green Goblin helmet for some time, and maybe 2026 is the year to finally put on the mask.