Microsoft probes boot failures after January KB5074109 Windows update

Pcgamer reports Microsoft is investigating customer reports that some devices fail to boot after installing the January 2026 Windows security update (KB5074109), released January 13, 2026, and later updates.

A Microsoft support document says it has received a "limited number of reports of an issue in which devices are failing to boot with stop code 'UNMOUNTABLE_BOOT_VOLUME'". Affected devices reportedly show a black screen with the message "Your device ran into a problem and needs a restart. You can restart." At that stage the device cannot complete startup and requires manual recovery steps. Microsoft also notes the problem has only been seen on physical devices, not virtual machines, and appears to affect systems on the 26200.7623 and 26100.7623 builds or later. The article also notes a separate surprise update on January 24 that patched an issue with apps becoming unresponsive when saving files to the cloud.

The issue is under investigation and Microsoft describes the reports as "limited", so how widespread the problem is remains unknown. You can check your OS build via "Settings" → "About" → "Windows info" to see if your device is on an affected build; the piece advises holding off on installing KB5074109 until Microsoft diagnoses the problem. If already affected, users must use the Windows Recovery option or perform a clean reinstall.

Key Topics

Tech, Microsoft, Windows, Unmountable_boot_volume, Windows Recovery