Microsoft has confirmed that its September 2026 security update silently broke Windows File History — the built-in backup tool millions of people rely on to recover deleted or corrupted files. A fix is now available, but it's an optional update that most Windows PCs will not install automatically until October's Patch Tuesday. If you use File History to back up to an external drive or network location, you need to check your PC now.

What broke

Many users who installed the September security patches for Windows found that the File History feature no longer works — deleted or corrupted files can no longer be recovered. File History automatically saves copies of files, allowing users to recover accidentally deleted or corrupted files using older versions that were stored hours, days, or weeks ago. By default, File History backs up all folders in the user profile (Documents, Music, Pictures, Videos, and Desktop) to an external drive, such as a USB flash drive or a network attached storage (NAS) device.

After the update, affected devices might incorrectly display a "Reconnect your drive" message even when a compatible backup drive is connected and functioning properly. Additionally, the "Last Backup" timestamp might not update, and previously backed up files might show "No previous version available." In some cases, Event Viewer might record application crash events referencing FileHistory.exe and KERNELBASE.dll.

According to Microsoft, the issue is due to a bug that occurs in all current versions of Windows 10 and 11, including 21H2, 22H2, 23H2, 24H2, 25H2, and 26H1. In other words, virtually every supported Windows PC is potentially affected.

How to tell if your PC has the bad update

The problematic update is Windows 11 KB5124008, released on September 8. If you see Windows 11 Build 26200.9445 in Settings > System > About, you've got the September patch. The equivalent build for Windows 11 24H2 is 26100.9445.

If File History was working before September 8 and now shows a "Reconnect your drive" error — or your backup timestamp hasn't updated in weeks — this bug is almost certainly the reason. That's a real problem if you were counting on those backups to recover lost files after a mistake or drive failure.

The fix (and why it won't install itself)

Microsoft released optional update KB5124010 for Windows 11 25H2 and 24H2, which brings several improvements but also fixes the File History issue. The same fix is also available via update KB5124006 for Windows 11 26H1.

Here's the catch: Both updates are optional Windows 11 updates that you must install manually; otherwise, the fix will roll out as part of October's Patch Tuesday. In other words, if you don't install this optional update, the File History bug will persist until next month.

To install the update, check Windows Settings → Windows Update → Advanced options → Optional updates.

This isn't the only September bug

The File History failure is part of a rough month for Microsoft. 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.

Owners of certain laptop brands have had it worse. On the HP side, there are instances of the update triggering BIOS corruption, leading to a failed install with no ability to recover the OS — though seemingly the OEM has published a new BIOS revision, so it's hard to say which side is to blame. Meanwhile, Lenovo users might have their machines black-screen or shut down out of the blue, seemingly due to power mismanagement. If your HP or Lenovo laptop stopped booting after the September update, that's a job for a laptop repair shop with recovery tools, not a home fix.

What to do right now

Three quick steps for anyone running Windows 10 or 11:

1. Check Settings → Windows Update → Advanced options → Optional updates and install KB5124010 (24H2/25H2) or KB5124006 (26H1) if you see them listed. 2. Open File History (Control Panel → System and Security → File History) and confirm the "Last Backup" timestamp is recent. If it's stuck on a date before September 8, your backups have been silently failing. 3. Don't rely on a single backup. Keep a second copy of anything important — family photos, tax documents, business files — on a separate drive or cloud service. If a drive fails before backups are restored, you'll need professional drive recovery to get your files back.

What This Means for York, PA

If you're in York County and File History is your only backup — or your HP or Lenovo laptop won't boot after the September patch — bring it into York Computer Repair at 2069 Carlisle Rd. We can verify your backups are actually running, install the optional fix cleanly, and pull data off drives that failed while backups were broken.

Windows 11 September Update Broke File History Backups — Fix Requires a Manual Install