Security1 publisher2 min readPublished
Microsoft: September 2026 update may stop File History from writing backups on some systems
Microsoft says some machines cannot create or update File History backups after installing the September 2026 security update, and affected PCs report the backup drive as disconnected while it sits attached and working.
The Watch · Security desk
What happened
- Microsoft warned that File History may stop working after the September 2026 Windows security updates, with affected customers unable to create or update backups of their profile folders.
- On affected machines, Event Viewer logs application crash events naming FileHistory.exe and KERNELBASE.dll.
- Windows may also display "Reconnect your drive" on those devices even when a compatible backup drive is attached and working properly.
- Files that were already backed up can report "No previous version available" when someone goes looking for an earlier copy.
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Why it matters
- exposure The machines that lose a recovery path are the ones where File History is the only versioned copy of a user's documents and pictures, and their owners are the least likely to open Event Viewer.
- constraint Backup verification cannot be done from the File History interface this cycle, because a working drive is reported as disconnected; it takes an event-log filter and a timestamp check on each machine.
- decision Anyone weighing a rollback is trading a month of security fixes against a profile-folder backup that a second product may already cover.
- precedent One out-of-band round has already failed to clear the September audio regressions, so a File History patch arriving will not by itself close out this update cycle.
The expensive part is detection. Windows blames the hardware, so the first hour of troubleshooting goes to cables, ports and a spare drive [3]. The event log answers faster. The process that dies is FileHistory.exe and the faulting module is KERNELBASE.dll, a core Windows library [2].
File History copies the folders under a user profile, Documents, Music, Pictures, Videos and Desktop, to an attached drive or a network location [6]. On a domain fleet with an agent-based backup product already running, that copy is redundant and this month's bug is a helpdesk cost. On a single machine where File History was the only versioned copy, the owner finds out at recovery time [5].
Microsoft's own wording is narrow. "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," the company said in a service alert seen by BleepingComputer and first spotted by Microsoft MVP Susan Bradley [7][8]. As reported, the alert includes neither a workaround nor a fix date [13].
September's cumulative updates have now produced five acknowledged regressions: File History, Remote Desktop Services, Hyper-V, USB audio and domain-credential sign-in [12]. Microsoft published a temporary workaround for the sign-in bug [11] and shipped out-of-band packages for the other three a week before the File History alert [9]. It then confirmed those emergency updates left some of the audio problems unfixed, with work still going on [10].
Until a fix ships, the Last Backup timestamp in the File History control panel is the cheapest per-machine check, and it stops advancing on affected devices [4].
What to watch
- Whether the File History fix arrives in the October 2026 cumulative update or as another out-of-band package.
- Whether the remaining September 2026 audio problems are resolved or carried into the next patch cycle.
- Whether Microsoft widens the File History alert to more Windows builds or names the originating KBs publicly.