Microsoft is investigating reports that some Windows 11 devices fail to boot with “UNMOUNTABLE_BOOT_VOLUME” stop errors after installing the January 2026 cumulative update KB5074109. Affected systems (Windows 11 25H2 and 24H2) display a black crash screen and require manual recovery; Microsoft is collecting Feedback Hub reports and separately issued OOB fixes for Outlook PST cloud freezes. #UNMOUNTABLE_BOOT_VOLUME #KB5074109
Keypoints
- Some Windows 11 devices fail to boot with “UNMOUNTABLE_BOOT_VOLUME” after the January 2026 updates.
- The issue affects Windows 11 version 25H2 and all editions of 24H2 following KB5074109.
- Affected systems show a black stop-error screen and require manual recovery to boot.
- Only physical devices have been reported as impacted; no virtual machines reported so far.
- Microsoft is investigating, asking users to submit feedback via Feedback Hub, and released separate OOB updates to fix Outlook PST cloud freezes.