Microsoft resolved a critical bug affecting some commercial systems running Windows 11 that caused boot failures due to an "UNMOUNTABLEBOOTVOLUME" error. This issue was linked to problematic updates from December 2025 and primarily impacted devices on Windows 11 versions 25H2 and 24H2. The resolution was included in the February 2026 Patch Tuesday update, specifically the Windows 11 KB5077181 security update released on February 10, 2026. An initial fix was provided in the optional update KB5074105 on January 29, 2026. Affected devices experienced failures after installing the January 13, 2026, security update KB5074109. Microsoft recommends that enterprise customers with still unbootable systems contact Microsoft Support for Business for assistance.