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.
Timeline
Jan 25, 2026
Microsoft ships emergency Outlook update for separate January issue
Microsoft released out-of-band emergency updates to fix a separate problem causing Outlook to freeze when PST files were stored in cloud services such as OneDrive or Dropbox. Reports noted that no equivalent fix was yet available for the Windows 11 unbootable-system issue.
Jan 25, 2026
Microsoft acknowledges and investigates the Windows 11 boot issue
Microsoft confirmed it was investigating reports that some physical Windows 11 devices fail to boot after the January 2026 update, while saying it had not seen the issue in virtual machines. The company asked impacted users and administrators to submit diagnostics through Feedback Hub as it worked to determine the root cause and scope.
Jan 25, 2026
Users report Windows 11 boot failures after installing KB5074109
After installing the January 2026 cumulative security update, some Windows 11 devices reportedly failed to boot and displayed an "UNMOUNTABLE_BOOT_VOLUME" blue screen error. Affected systems could not complete automatic restart and required manual recovery steps such as using the Windows Recovery Environment to remove the update.
Jan 13, 2026
Microsoft releases January 2026 Windows 11 security updates
Microsoft began rolling out the January 2026 Patch Tuesday security updates, including cumulative update KB5074109, starting on 2026-01-13. The update later became linked to boot failures on some Windows 11 24H2 and 25H2 systems.
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