Microsoft says it has resolved a Windows 11 bug that caused some commercial systems to fail to boot with an “UNMOUNTABLE_BOOT_VOLUME” error after installing recent security updates, with the fix delivered in the February 2026 Patch Tuesday update.
The boot issue, which Microsoft previously investigated and linked to failed December 2025 updates, affected a limited number of commercial Windows 11 devices running versions 25H2 and 24H2.
According to a private enterprise advisory seen by Susan Bradley of Ask Woody, the issue has now been marked as fully resolved in the Windows 11 KB5077181 security update released on February 10, 2026.
Microsoft says impacted devices suffered boot failures after installing the January 13, 2026, security update KB5074109 or later updates, displaying a black screen and the message: “Your device ran into a problem and needs a restart. You can restart.”
At that point, impacted systems were unable to boot and required manual recovery to restore functionality.
Microsoft previously confirmed the issue was caused by the failed installation of the December 2025 security update, leaving devices in an improper state after the installation rolled back.
Attempting to install future Windows updates on devices with this “improper state” could cause the system to become unbootable.
Microsoft said the issue affected only physical devices running Windows 11 25H2 and 24H2, and did not receive reports of it affecting home users or virtual machines.
Fix delivered in February Patch Tuesday update
Microsoft says it previously released an initial resolution in the optional non-security preview update KB5074105 on January 29, 2026, which helped prevent additional systems from becoming affected by the bug.
The company now says the issue is fully resolved in the Windows 11 KB5077181 update released during the February 2026 Patch Tuesday and later updates.
“This issue is fully resolved in the Windows security update released on February 10, 2026 (the Resolved KBs listed above), and later updates,” reads Microsoft’s advisory.
Unfortunately, devices that became unbootable before the fix was released may still require additional remediation.
Microsoft advises enterprise customers whose devices remain affected to contact Microsoft Support for Business for assistance restoring system stability.
It is unclear why Microsoft did not share this advisory publicly, as it does for other known Windows issues.
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