Microsoft probes Windows 11 boot failures tied to January security updates

Microsoft is investigating reports that its January 2026 security updates are leaving some Windows 11 machines stuck in a boot loop, adding another entry to this month's bumper post–Patch Tuesday borkage list.

The issue affects a "limited number" of physical Windows 11 devices that fail to start after installing the January security updates and later patches, instead showing an "UNMOUNTABLE_BOOT_VOLUME" stop code and a black screen offering little more than a restart prompt.

"Your device ran into a problem and needs a restart. You can restart," the message reads, according to a Microsoft advisory spotted by Ask Woody. At this stage, Microsoft says, the system cannot complete startup and needs manual recovery steps to get going again.

According to Redmond's own incident note, the problem has been observed on Windows 11 versions 24H2 and 25H2, with no server editions affected and no reports of the same behavior ...