Windows 11’s January 2026 cumulative update, KB5074109, rolled out as a mandatory Patch Tuesday security release — but it’s quickly become associated with a swath of problems for some systems. The update was intended to deliver routine security fixes and quality improvements, yet users and administrators are now reporting cases where devices fail to boot altogether after installing it.
According to Microsoft’s own advisory, a limited number of Windows 11 PCs running versions 25H2 and 24H2 have stopped at an early stage of the startup process following the update, showing a black screen with a UNMOUNTABLE_BOOT_VOLUME error code (0xED) and unable to reach the desktop. These failures appear isolated to physical hardware rather than virtual machines, but the exact conditions triggering them haven’t been confirmed. Recovery often requires manual intervention via the Windows Recovery Environment or, in the worst cases, a clean reinstall of Windows.
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Beyond boot issues, the KB5074109 update has been linked to a range of secondary glitches. Reports include app crashes, black screens, GPU-related performance problems, and Outlook freezing on POP or PST file access, prompting Microsoft to issue emergency out-of-band patches for some regressions.
Guide articles and community threads suggest that uninstalling the problematic update may alleviate many symptoms, especially on systems where core functions like shutdown or remote desktop logins are disrupted. However, some users find the uninstall process itself fails with errors such as 0x800f0905, leaving them caught between a flawed update and an incomplete rollback.
For now, Microsoft continues to investigate the root causes and collect telemetry from affected devices. Users are advised to consider pausing updates until fixes are released, and those facing issues should consult support resources for recovery options.