Microsoft has fixed a known issue that breaks the built-in File History backup feature on some Windows systems after installing the September 2026 security updates.
File History, introduced in Windows 8 and later replaced by Windows Backup, automatically backs up 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) and then lets them restore accidentally deleted or damaged files using an older version from hours, days, or weeks ago.
In a Saturday service alert seen by BleepingComputer, Microsoft warned that the automated backup feature might stop working after installing this month's security updates and triggering application crash events in Event Viewer referencing FileHistory.exe and KERNELBASE.dll.
On impacted systems running Windows 10 21H2 or later, Windows 10 Enterprise LTSC 2016/2019, and Windows 11 23H2 and later, users are also seeing "Reconnect your drive" alerts even though compatible backup drives are connected and working properly.
Other symptoms include previously backed-up files showing "No previous version available" and the "Last Backup" timestamp not updating.
"After installing the September 2026 Windows security update [..], some customers using File History, might be unable to create or update backups," Microsoft 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."
Fixed in September 2026 preview update
On Tuesday, Microsoft said the known issue has been fixed for Windows 11 customers and is rolling out with this month's KB5124006 and KB5124010 optional cumulative updates for devices running Windows 11 26H1 and Windows 11 24H2/25H2, respectively.
"This update addresses an issue where File History might fail to back up or restore files to an external drive or network location," Microsoft said.
"We recommend you install the latest update for your device as it contains important improvements and issue resolutions, including this one."
Windows users who don't want to install the optional updates will get the fix with next month's Patch Tuesday updates, which will roll out on October 13.
One week ago, Microsoft also released out-of-band updates to fix Hyper-V problems, Remote Desktop Services failures, and USB audio issues triggered by the September Patch Tuesday security updates.
Days later, it also shared a temporary fix for a bug that prevents some Windows 11 users from logging in with valid domain credentials after installing the same buggy updates.
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