Microsoft has confirmed that its January 13, 2026 cumulative update for Windows 11 (KB5074109) is tied to a serious, if currently limited, regression that can leave physical PCs unbootable with the stop code UNMOUNTABLE_BOOT_VOLUME — a black-screen failure that requires manual recovery through WinRE or, in worst cases, a clean install.
Background / Overview
Microsoft shipped its January 13, 2026 Patch Tuesday cumulative update (tracked as KB5074109, OS builds 26200.7623 and 26100.7623) to...