"Microsoft has received a limited number of reports of an issue in which devices are failing to boot with stop code 'UNMOUNTABLE_BOOT_VOLUME', after installing the January 2026 Windows security update (the Originating KBs listed above), released January 13, 2026, and later updates," Microsoft stated.

The company says that affected devices will show a black Windows crash screen stating, "Your device ran into a problem and needs a restart. You can restart."

Microsoft says the reported systems are unable to boot into Windows and require manual recovery efforts to boot again.