Microsoft Investigates Windows 11 Boot Failures in January
⚠️Microsoft is investigating reports that some Windows 11 devices fail to boot with the UNMOUNTABLE_BOOT_VOLUME stop error after installing the January 13, 2026 cumulative update KB5074109. Affected systems running Windows 11 25H2 and all editions of 24H2 display a black crash screen and cannot start without manual recovery. Microsoft says only physical devices are impacted so far and asks affected users to submit feedback via the Feedback Hub. The company also released emergency out‑of‑band updates to address an Outlook PST cloud storage freeze.
