Overview
- Some Windows 11 systems show the stop code UNMOUNTABLE_BOOT_VOLUME after the January 13 security updates, preventing startup.
- Microsoft characterizes the reports as limited and has not detailed a root cause, leaving the overall prevalence unclear.
- An out-of-cycle cumulative update restores stability for applications and fixes errors opening or saving files with cloud storage such as OneDrive and Dropbox.
- The update also resolves Outlook hangs when PST files reside on OneDrive and is available across multiple Windows client and server versions, retaining January security fixes.
- Separately, Microsoft has begun replacing Secure-Boot KEK certificates, swapping the 2011 KEK for the 2K 2023 KEK, a maintenance action that may prompt restarts and is not believed to be related to the boot issue.