Windows 11 KB5074109 (January 2026 Update) is causing major issues on some PCs, and Microsoft has more details to share. In an updated support document, the company has confirmed that the update is indeed crashing some PCs, but most home users won’t run into the problem, as the bug is currently limited to commercial PCs.

Microsoft has already taken steps to ensure it does not affect consumers.

On January 23, Microsoft confirmed that it was aware of reports that some PCs do not boot anymore.

At that point, Microsoft said it suspected that some PCs were not booting, but could not verify whether the issue was due to a Windows update. Now, the company has confirmed that a limited number of PCs are failing to boot after the update. This is a known issue and is now under active investigation.

This issue affects PCs running Windows 11 KB5074109 (25H2 Build 26200.7623 and 24H2 Build 26100.7623).

If you were affected, you’d see a Black Screen of Death (BSOD). For those unaware, Microsoft recently redesigned the Blue Screen of Death as “Black Screen,” so it matches the look and feel of Windows 11. That’s why you’ll no longer see the original blue screen or the frowning face.

The Black Screen of Death (BSOD) appeared with UNMOUNTABLE_BOOT_VOLUME error and required customers to manually recover their computer. That meant it is no longer possible for affected PCs to boot. To recover the PC, Microsoft said you would need to enter WinRE (Windows Recovery Environment) and use one of the available methods.

Microsoft now says it has identified the root cause in only some commercial PCs (non-consumer) and that it’s due to a Windows Update, not a driver or third-party software.

The January 2026 update didn’t “randomly” brick healthy PCs

According to the company, the no-boot (UNMOUNTABLE_BOOT_VOLUME) issues are happening mostly on machines that were already in a bad state because they failed to install the December 2025 security update, and then rolled it back.

That rollback left the system in an “improper state”, but those PCs were still in a working state until Windows 11 KB5074109 automatically installed. Most likely, some combination of servicing metadata, boot configuration, or disk or driver state ended up half-applied or inconsistent.

On a normal PC, Windows updates assume the system is in a valid baseline state. However, that wasn’t the case with these affected PCs.

When those “improper state” devices tried to install the Windows 11 KB5074109 (or later updates), the update process had to touch low-level components. Because the baseline was already inconsistent, the new update could push it over the edge into a condition where Windows can’t mount the system volume during startup

That’s when you get UNMOUNTABLE_BOOT_VOLUME (0xED), and the machine can’t complete boot without manual recovery steps.

“Recent investigations have determined this issue can occur on devices that failed to install the December 2025 security update and were left in an improper state after rolling back the update,” Microsoft noted in a support document.

“Attempting to install Windows updates while in this improper state could result in the device being unable to boot. We are working on a partial resolution that will prevent additional devices from resulting in a no-boot scenario if they try to install an update while in this improper state. However, this partial resolution will not prevent devices from getting into the improper state in the first place, nor will it repair devices that are already unable to boot. We continue to investigate why these devices are failing to install Windows updates or potentially getting into this improper state,” the company explained.

Microsoft has already deployed a fix that would prevent additional devices from crashing. This means consumers will now never run into this specific issue, at least not until another bug hits Windows.

January 2026 Patch Tuesday has other issues

This month’s first update has tons of problems, including broken Outlook (POP and PST issues). Other issues include broken customization in File Explorer, sleep mode (S3) not working on older PCs, Remote Desktop login failure, and shutdown failure, among other minor issues.

Microsoft has already rolled out an emergency update (KB5078127) to patch Outlook issues and is working on other problems.

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