Microsoft warned that the built-in File History backup feature in Windows may stop working on some systems after installing the September 2026 security updates.

File History (introduced in Windows 8 and replaced by Windows Backup, which backs up data to OneDrive) automatically saves copies that let users recover accidentally deleted or damaged files using an older version from hours, days, or weeks ago.

By default, File History backs up all folders under the user's account profile (Documents, Music, Pictures, Videos, and Desktop) to an external hard drive, such as a USB flash drive, or a network storage location (NAS).

In a service alert seen by BleepingComputer and first spotted by Microsoft MVP Susan Bradley, Microsoft says that affected customers can see application crash events in Event Viewer referencing FileHistory.exe and KERNELBASE.dll.

"After installing the September 2026 Windows security update (the Originating KBs listed above), some customers using File History, might be unable to create or update backups," it said. "File History, available through Control Panel > System and Security > File History, is used to back up files to an external drive or network location."

On affected devices, Windows might also incorrectly display "Reconnect your drive" messages even when compatible backup drives are connected and working properly. Other symptoms include the "Last Backup" timestamp not updating and previously backed-up files showing "No previous version available."

Affected Windows releases include Windows 10 21H2 or later, Windows 10 Enterprise LTSC 2016/2019, and Windows 11 23H2 and later.

The September 2026 cumulative updates have been linked to multiple issues affecting home and enterprise Windows users.

One week ago, Microsoft released out-of-band updates to fix Remote Desktop Services failures, Hyper-V issues, and USB audio problems triggered by this month's security updates.

However, it also confirmed that these emergency updates did not resolve all the audio problems introduced by the September 2026 updates, and that it is still working on a fix for the remaining issues.

Microsoft also shared a temporary workaround for a separate bug that prevents some Windows 11 users from logging in with valid domain credentials after installing the September updates.

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