Microsoft linked recent Windows 11 boot failures after the January 2026 update to devices that previously failed to install the December 2025 security update and were left in an “improper state.” Attempting to install updates while in that state can produce an UNMOUNTABLE_BOOT_VOLUME BSOD and Microsoft is working on a partial fix that won’t repair already-unbootable devices. #Windows11 #KB5074109
Keypoints
- Microsoft traced the January 2026 boot failures to earlier failed installs of the December 2025 security update.
- Rolling back the December 2025 update left some devices in an “improper state” that blocks safe updating.
- Installing KB5074109 on affected Windows 11 25H2 and 24H2 systems caused UNMOUNTABLE_BOOT_VOLUME BSODs.
- Microsoft is deploying a partial resolution to prevent new no-boot cases but it won’t fix devices already unable to boot or stop the initial improper state.
- The issue has been reported on physical devices only, with no virtual machine impacts so far.