Microsoft has fixed a known issue that stopped the File History backup feature from working on some Windows PCs after the September 2026 security updates were installed.

The fix ships in the optional KB5124010 and KB5124006 cumulative updates for Windows 11, according to Microsoft, and Windows 11 users who skip those optional updates will receive it with the October 13 Patch Tuesday release.

Microsoft first warned about the problem in a Saturday service alert on its Windows release health dashboard. On affected systems, File History stops creating or updating backups, and Event Viewer records application crash events that reference FileHistory.exe and KERNELBASE.dll.

The bug reaches well beyond the newest releases. Microsoft lists Windows 10 21H2 and later, Windows 10 Enterprise LTSC 2016 and 2019, and Windows 11 23H2 and later as affected.

Failed backups are not always obvious. Users see "Reconnect your drive" alerts even when a compatible backup drive is connected and working properly. Files that were backed up earlier can show "No previous version available," and the "Last Backup" timestamp stops updating.

"After installing the September 2026 Windows security update [..], some customers using File History might be unable to create or update backups," Microsoft said in the alert.

File History, found under Control Panel > System and Security > File History, copies the Documents, Music, Pictures, Videos, and Desktop folders of a user profile to an external drive or a network location, and lets users restore older versions of deleted or damaged files. The feature was introduced in Windows 8 and was later replaced by Windows Backup.

Microsoft confirmed on Tuesday that the fix is rolling out with this month's optional preview updates. According to the release notes, the update "addresses an issue where File History might fail to back up or restore files to an external drive or network location." The company recommends installing the latest update for each device.

1. Open Windows Update and check for available updates.
2. Install KB5124010 on Windows 11 24H2 or 25H2, or KB5124006 on Windows 11 26H1.
3. Restart the PC when the installation finishes.
4. Open Control Panel > System and Security > File History and confirm that the "Last Backup" timestamp changes after the next backup.

Optional updates are preview releases, so administrators who prefer to wait for Patch Tuesday can skip them and pick up the same fix on October 13. Until then, File History users on unpatched systems should not rely on the feature as their only backup.

File History is not the only feature hit by this month's patches. Microsoft released out-of-band updates last week for Hyper-V, Remote Desktop Services, and USB audio problems, and September updates also broke Always On VPN connections.

Microsoft's fix announcement covers Windows 11, and the company has not listed a separate fix for the affected Windows 10 releases.