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 versions | Originating update |
|---|---|
| Windows 11, version 26H1 | KB5124012 |
| Windows 11, version 25H2 | KB5124008 |
| Windows 11, version 24H2 | KB5124008 |
| Windows 11, version 23H2 | KB5122880 |
| Windows 10, version 22H2 | KB5122878 |
| Windows 10, version 21H2 | KB5122878 |
| Windows 10 Enterprise LTSC 2019 | KB5122876 |
| Windows 10 Enterprise LTSC 2016 | KB5123099 |