Microsoft has confirmed a Windows 11 problem that should get immediate attention from anyone relying on File History for local backups. After the September 2026 security update, some PCs can no longer create or update File History backups, and the failure may not be obvious unless you check the backup status yourself.

The affected update is KB5124008, the September 8 security release for Windows 11 24H2 and 25H2. According to Microsoft’s acknowledgement, File History users may see backup creation or updates fail after installing it. Windows Latest also reports that affected systems can show a misleading “Reconnect your drive” warning even when the external disk or network backup target is attached and working normally.

For home users, that is inconvenient. For IT teams, consultants, and power users who assumed File History was quietly protecting desktop data, it is a risk. A backup tool that fails silently is worse than one that fails loudly, because it can create false confidence right up until a restore is needed.

What appears to be broken

File History is the classic Windows feature that backs up user libraries and selected folders to an external drive or network location. It is not a full system image solution, but it is still useful for recovering earlier versions of documents, pictures, desktop files, and other personal data.

The September bug can interrupt that protection. Microsoft says some customers using File History may be unable to create or update backups after KB5124008. Windows Latest adds that the user interface may incorrectly claim that the backup drive needs to be reconnected, and that the “Last backup” timestamp may be stale or wrong. In some cases, Event Viewer may record crashes involving FileHistory.exe and KERNELBASE.dll.

The practical takeaway is simple: do not assume File History is working just because it was configured before September’s patch cycle. Treat backup status as unverified until you see a fresh File History run complete and confirm that recent files are actually present on the backup destination.

Who should check first

This issue matters most for users who depend on File History as their primary local backup. That includes small offices backing up to USB disks, home users with external drives, and anyone storing important working files outside cloud-synced folders.

Administrators should also pay attention to shared or managed PCs where File History was used as a lightweight safety net rather than a centrally monitored backup system. If users rarely open the File History control panel, the problem may go unnoticed for days or weeks.

Systems using separate enterprise backup software are not automatically affected in the same way, but it is still worth verifying scheduled backup jobs after this month’s Windows updates. September’s Windows 11 update cycle has included several unrelated regressions, including Remote Desktop issues, USB audio problems, Explorer crashes, and domain sign-in failures in specific environments.

What to do now

Start by checking File History directly. Open Control Panel, go to System and Security, then File History. Confirm whether Windows reports a recent successful backup. If it asks you to reconnect a drive that is already available, do not immediately assume the drive is bad. Check whether the same disk or network path opens normally in File Explorer.

Next, run a manual backup if the option is available. Then browse the backup target and confirm that recent files have been updated. The timestamp shown in Windows may not be reliable on affected systems, so validate the actual backup contents rather than relying only on the interface.

If File History is failing, use a temporary second backup method until the fix is installed. For individual users, that could mean copying important working folders to another external drive, enabling OneDrive Known Folder Move for Desktop, Documents, and Pictures, or using another trusted backup application. For business users, the better answer is a centrally monitored backup product that reports failures rather than depending on users to notice them.

Should you install the optional fix?

Windows Latest reports that Microsoft’s emergency September update KB5129195 fixed some other problems but did not resolve the File History bug. The newer optional preview update, KB5124010, is reported to include the File History fix for Windows 11 24H2 and 25H2.

That creates the usual Windows servicing trade-off. Optional preview updates can deliver fixes earlier, but they can also introduce new problems because they are not the regular Patch Tuesday release. If File History is business-critical and currently broken, testing KB5124010 on a small number of machines may be reasonable. If you can use an alternative backup method for a few weeks, waiting for the next cumulative update may be safer.

For managed environments, avoid a blanket deployment without testing. Verify the fix on representative hardware, especially if the September update already caused other issues in your environment. Also document which machines relied on File History so you can confirm backup recovery after patching.

Bottom line

The immediate risk is not that Windows deleted existing File History backups. The risk is that new backup versions may not be created after the September update, leaving users with an older recovery point than expected. That gap matters if a file is deleted, overwritten, encrypted by malware, or lost during hardware failure.

Until Microsoft’s fix is applied and verified, File History users should manually confirm that current backups exist. A backup strategy is only useful when it is tested, current, and restorable. This bug is a reminder to check all three.

Source: Windows Latest