Microsoft has announced that it has fixed a serious issue in Windows 11that was causing to fail to boot some commercial systems after installing recent security updates. The error was displayed with the message “UNMOUNTABLE_BOOT_VOLUME,” and the company says the final fix was released with the February 2026 Patch Tuesday.
The issue had been around since late 2025, as it was linked to failed December updates that left some devices in an unstable state. Although it affected a limited number of corporate computers, the problem was considered particularly critical because it led to a complete inability to boot the operating system.
See also: Microsoft strengthens Windows security with new default protections
Which versions of Windows 11 were affected?
According to Microsoft, the bug appeared on physical commercial devices running Windows 11 builds 25H2 and 24H2. The company clarified that there were no reports of impact to home users or virtual machines, which limits the scope of the incident mainly to corporate environments.
The issue became more apparent after installing security update KB5074109 on January 13, 2026 or later. Affected devices would experience a black screen and an error message informing the user that the system encountered a problem and needed to be restarted.
However, in many cases, restarting did not lead to recovery, but to a complete inability to load Windows, requiring a manual recovery process.
The cause: Failed installation of December 2025 update
Microsoft had previously confirmed that the root of the problem was a failed installation of the security update . When the installation failed and a rollback was performed, some devices were left in an "inappropriate system state."
See also: Microsoft will add Sysmon natively to Windows 11
This meant that any attempt to install future updates could result in a catastrophic boot error, as the boot disk could not be properly mounted by the operating system.
This type of error is particularly serious in corporate environments, where loss of access to workstations can cause delays, downtime, and increased support costs.
The solution via Patch Tuesday February 2026
Microsoft had initially tried to limit the spread of the problem by releasing a temporary fix in the optional preview update KB5074105 on January 29, 2026. This update helped prevent more systems from being affected.
The full fix, however, came with security update KB5077181, which was released on February 10, 2026 as part of Patch Tuesday.
The company clearly states that the problem has now been fully resolved in this update and all subsequent versions.
What should affected businesses do?
Although the fix has been released, Microsoft warns that devices that were already crashed before the update may require additional recovery procedures.
See also: Microsoft disables NTLM protocol in Windows
For this reason, enterprise customers who continue to experience boot failures are urged to contact Microsoft Business Support for guidance on restoring system stability.
Microsoft Contact Questions
An interesting point is that Microsoft did not publicly communicate this notice in the way it usually does for other known Windows issues. This raises questions about transparency in informing users, especially when it comes to bugs that can lead to complete loss of access to the operating system.
The incident is yet another reminder that security updates, while necessary, can create serious challenges, especially in complex enterprise environments where stability is as critical as security.
Source: www.bleepingcomputer.com
