Windows 11 KB5074109 update now linked to boot failures

Microsoft’s January 2026 Windows 11 security update KB5074109 is now being linked to a more serious issue than crashes and app errors: some PCs are failing to boot entirely with a UNMOUNTABLE_BOOT_VOLUME blue screen.

According to Microsoft’s internal status notes, summarized by community tracking at AskWoody and recent reports from BleepingComputer and Windows Central, a “limited number of reports” have surfaced where Windows 11 devices on version 24H2 and 25H2 hit a UNMOUNTABLE_BOOT_VOLUME stop code after installing the January 13 security updates based on KB5074109.

In these cases, systems never reach the login screen and instead loop into a BSOD, leaving users to recover the machine from the Windows Recovery Environment (WinRE) or external media.

Microsoft confirms reports but has no fix yet

Microsoft says it is investigating the boot failures and, so far, is treating them as affecting only a small subset of devices. The company has not yet confirmed the exact root cause or published a dedicated hotfix for the UNMOUNTABLE_BOOT_VOLUME issue, and its public release-health pages still focus mainly on Remote Desktop problems, black screens, and cloud app crashes tied to KB5074109 and the January out-of-band patches.

The boot failures arrive on top of:

• Black screen and app crash issues after KB5074109
• Outlook Classic instability and OneDrive/Dropbox hangs, which triggered emergency updates KB5077744 and KB5078127
• Cases where uninstalling KB5074109 fails with error 0x800f0905, leaving users stuck between a bad update and a broken rollback path

What can Windows 11 users can do

For now, Microsoft’s guidance for UNMOUNTABLE_BOOT_VOLUME is the same as for other boot failures: use WinRE and standard recovery tools to repair or roll back the system. In practice, that means booting from the recovery environment or installation media, attempting file system repair, and, where possible, uninstalling the January security update from recovery before Windows loads.

For users whose PCs are still booting normally after KB5074109, there’s no need to panic, but this new stop-code issue underlines how fragile the January Windows 11 update cycle has become. Between emergency hotfixes, rollback errors, and now full boot failures, many admins will be treating KB5074109 with extra caution until Microsoft delivers a fully stabilized cumulative update.