Microsoft is investigating an issue where some Windows 11 systems no longer start correctly after the January 2026 security updates.
After installing the January Windows security updates, a limited number of PCs get stuck in a boot loop. Users see a black screen with the error message “UNMOUNTABLE_BOOT_VOLUME”, after which they have to restart their PC. According to Microsoft, the system cannot fully start in these cases and manual recovery steps are required.
Limited impact, multiple versions affected
The issue affects Windows 11 versions 24H2 and 25H2. Windows Server and virtual machines do not appear to be affected. The first reports appeared on user forums, where people indicated that systems that used to work fine would no longer start after the updates.
Microsoft claims to have received only a small number of reports for the time being, The Register reports. The company is investigating whether the cause is effectively due to problems in the January patches. Affected users are advised to contact business support or submit feedback via the Feedback Hub.
January update cycle under pressure
The startup error comes on top of other problems that surfaced this month. Microsoft already had to intervene with a Secure Launch-bug and Outlook freezing when saving to cloud storage.