Windows 11 boot failures officially blamed on a chain of bad updates

Microsoft has now confirmed that the Windows 11 KB5074109 January 2026 security update is not “randomly” bricking healthy PCs, but triggering boot failures on machines that were already left in an improper state by a failed December 2025 security update. Devices that attempted the December update, rolled it back, and continued running on that broken baseline are now more likely to encounter a UNMOUNTABLE_BOOT_VOLUME (0xED) stop error and a black screen after installing KB5074109.

Scope of impact:

According to Microsoft, the issue affects a limited number of physical Windows 11 24H2 and 25H2 devices, mostly commercial PCs, and does not appear to impact virtual machines. The company stresses that the January update is effectively “stacking” on top of a corrupted state created in December, which is why only some systems end up in no-boot loops while others install KB5074109 without incident.

Microsoft says it is working on a partial resolution that will prevent additional devices from entering a no-boot state once they are already in that improper state. However, this fix will not repair PCs that are already unable to boot, nor will it retroactively fix the underlying December failure. Affected machines still need manual recovery via the Windows Recovery Environment or external media to uninstall updates or repair the OS.

WinRE-based recovery

For now, the guidance remains the same: admins are advised to check update history for failed December 2025 installs, avoid pushing KB5074109 to at-risk fleets, and rely on WinRE-based uninstall or full reinstalls for systems already stuck on the UNMOUNTABLE_BOOT_VOLUME screen. 

The new explanation doesn’t end the Windows 11 update saga, but it does give the first clear picture that January’s boot failures are the result of a chain of bad updates, not a single isolated patch.