Windows 11 January Security Updates Trigger UNMOUNTABLE_BOOT_VOLUME Boot Failures

Microsoft is investigating and has acknowledged a limited issue where some Windows 11 devices fail to boot after installing the January 2026 Patch Tuesday security updates, presenting a BSOD/black crash screen with stop code UNMOUNTABLE_BOOT_VOLUME. Impacted systems can become stuck in a restart loop and are unable to start Windows without manual recovery efforts, with Microsoft collecting reports from users and enterprise administrators to determine scope and root cause.

Reporting indicates the problem affects physical devices (with no virtual machines reported as impacted so far) and is tied to specific Windows 11 builds and cumulative updates, including Windows 11 25H2 and Windows 11 24H2 after installing KB5074109. Microsoft has not yet confirmed the underlying cause or provided a universal remediation beyond recovery steps, and is requesting affected customers submit diagnostics via the Feedback Hub while it determines whether the behavior is a regression introduced by the update.