Windows 11: January 2026 update causes blue pants and reboot in physical PCs (UNMOUNTABLE _ BOOT _ VOLUME)

If in the last few days you have updated Windows 11 and your computer does not start, you are not the only one. Microsoft has recognized that a limited number of physical devices are experiencing an error screen when starting after applying the January 2026 security updates, associated with the cumulative identified as KB5074109. The most common symptom is the known blue pants with the stop code UNMOUNTABLE _ BOOT _ VOLUME which prevents the system from mounting the boot partition and forces the team to reboot again and again.

According to the information shared by the company itself, the machines concerned correspond to Windows 11 version 25H2 and all the editions of the 24H2 version after installing that specific package. Microsoft describes the incident as "a limited number of reports" and clarifies that, for now, the failures have been detected only in physical equipment; there are no confirmed reports of virtual machines with this problem. You can see the general health status of Windows versions on Microsoft's official Windows Release Health page: https: / / learn.microsoft.com / windows / releases /.

The failure is presented in a clear way: when the computer is turned on, the black screen appears with the message "Your device ran into a problem and needs a restart. You can restart." And the system can't complete the boot. This requires the use of manual recovery procedures to return the machine to a functional state. Microsoft is actively investigating whether this regression was caused by the update itself and has promised to update its documentation when it has more details.

If your computer is affected, Microsoft requests users and administrators to send information through the Feedback Hub application to help diagnose the problem: sends feedback to Microsoft team. This shipment is useful because adding reproducible records and steps can accelerate the location of the cause by the engineers.

From a technical point of view, the UNMOUNTABLE _ BOOT _ VOLUME error code is related to the system's inability to mount the partition containing the files needed to boot Windows. Microsoft maintains technical documentation on these types of errors that can serve as a reference if you are going to try to recover: Bug Check 0x0000ED - UNMOUNTABLE _ BOOT _ VOLUME.

If you have not yet installed the update and manage devices in a company or are a concerned user, the most prudent thing is to temporarily pause or block the installation of the patches in production equipment until Microsoft confirms the cause and publishes a solution. For managed environments, tools such as Windows Update for Business, WSUS or Intune allow you to control the deployment and postpone critical updates while it is being investigated.

For those already affected, the usual recovery options go through using the Windows Recovery Environment (WinRE) to run the automatic repair, restore the system to an earlier point or run disk checks (chkdsk) and boot manager repair utilities. These actions require technical experience and, in teams with important data, it is wise to consult with the technical support of your organization or with a professional service before trying repairs that can complicate the recovery of information. Microsoft offers resources and tools to create installation or recovery means from its official page: Create Windows 11 Installation Media.

Parallel to the boot problem, Microsoft released off-of-band updates on the weekend to correct another incident: a Microsoft Outlook lock when PST files were stored in cloud services such as OneDrive or Dropbox. According to the company, this failure mainly affected business scenarios and components used by applications in corporate environments. If you manage mail environment in an organization, you should review the Office update notes and apply the Microsoft recommended corrections; documentation on Office and Microsoft 365 updates is available at https: / / learn.microsoft.com / officeupdates.

What can you do now? First, keep calm: not all teams were affected and the incidence has been rated by Microsoft as limited in range. Second, if you administer several PCs, stop the automatic deployment of the affected update and coordinate a quarantine until you have more information. Third, for the already blocked equipment, collect as many details as possible (brand and model of the computer, exact version of Windows, if there were third-party software of disks or active encryption, and the steps that led to the failure) and register them in Feedback Hub to help Microsoft.

Finally, remember that keeping regular backup and always accessible means of recovery is the best protection against such surprises. Microsoft will continue to publish updates and notices on its official channels; if you want to follow the incident development, often consult the Windows status page and technical support releases.

If you need to, I can help you write a message for your IT team with the recommended urgent actions, or guide you step by step to collect logs and useful information to send from Feedback Hub. Just tell me what kind of equipment you have and whether it belongs to a personal or corporate environment.