Overview
- Microsoft says affected PCs were left in an improper state after a failed rollback of the December 2025 security update.
- The company has deployed a partial resolution to prevent additional devices from entering a no‑boot state during updates, which neither repairs bricked systems nor stops devices from reaching the improper state.
- Impacted machines show an UNMOUNTABLE_BOOT_VOLUME error on a black crash screen and require recovery via the Windows Recovery Environment.
- Failures have been reported after installing KB5074109 on Windows 11 versions 24H2 (Build 26100.7623) and 25H2 (Build 26200.7623).
- Microsoft says the issue is limited to physical commercial devices with no reports of virtual machines being affected and attributes the fault to Windows Update rather than drivers or third‑party software.