Microsoft links Windows 11 boot failures to failed December 2025 update

Microsoft reports that some Windows 11 devices experienced boot failures (UNMOUNTABLE_BOOT_VOLUME) after installing the January 2026 cumulative update (KB5074109) due to being left in an “improper state” following failed attempts to install the December 2025 security update. The company is rolling out a partial mitigation to prevent additional devices from entering a no-boot state during updates, though it will not fix systems already affected or stop devices from entering the improper state initially. The issue currently appears limited to physical machines, with no impact observed on virtual machines, and investigation is ongoing.