Windows 11 September Update Breaks File History Backups

Microsoft has added a new known issue to its September 2026 Windows servicing documentation: some PCs using File History can no longer create or update backups after installing the September security updates. On Windows 11 versions 24H2 and 25H2, the affected servicing line begins with KB5124008, released September 8.

This is more consequential than a cosmetic Control Panel error because File History is a versioned local backup feature. Microsoft says affected systems can show a “Reconnect your drive” warning even while a compatible external or network backup destination remains connected and otherwise functional. The Last Backup timestamp may stop advancing, and previously backed-up files can incorrectly appear to have no previous version available.

Microsoft issued the cumulative out-of-band KB5129195 update for Windows 11 24H2 and 25H2 on September 14 to resolve other September regressions, including Remote Desktop Services failures and Plan9 shared-folder problems affecting some Hyper-V-based Linux environments. Because the emergency update is cumulative, however, installing it does not roll the File History regression back.

Microsoft still lists File History as an open known issue and says it is working on a resolution for a future Windows update. That makes the servicing distinction important: a PC can be fully updated through the September out-of-band release and still be affected by this particular backup problem.

The false reconnect warning makes this regression easy to misdiagnose as a USB drive, NAS, cable or filesystem problem. Those failures are still possible, but Microsoft explicitly says the warning can appear while a compatible backup drive is connected and functioning properly. That is a reason to check the Windows update state and File History timestamps before replacing hardware or rebuilding a backup destination.

It is equally important not to overstate the scope. Microsoft describes the issue as affecting some customers using File History; it does not say every Windows 11 installation or every backup method is broken. File History is also distinct from OneDrive-backed Windows Backup and from third-party backup software, so the confirmed issue should not be generalized to those systems without separate evidence.

The safest immediate response is verification rather than a destructive workaround. Check whether File History’s Last Backup time is advancing and whether recent file versions can actually be browsed. If the feature is failing, preserve the existing backup media and use another known-good backup path for new important data until Microsoft ships a documented resolution.

Microsoft has not published a File History-specific fix in the current Windows 11 servicing notes, so Core Tech Tips is not recommending removal of security updates as a routine solution. September’s updates contain security fixes, and uninstalling a cumulative security update trades one documented backup regression for a different security posture. The useful milestone to watch is Microsoft marking this known issue resolved in a later servicing release.