Windows 11 File History Says “Reconnect Your Drive”? KB5124008 Broke It — Here’s the Fix (2026)

File History didn’t crash loudly. It just stopped, and kept telling people to plug in a drive that was already plugged in. That’s what makes this one dangerous — anyone who dismissed the notification has been running without backups since early September and may not know it.

The fix itself is easy. The part most guides skip is working out how big your backup gap is, and whether the optional update is worth its own two known issues.

Contents

• What Did KB5124008 Break in File History?
• How Do You Know If Your PC Is Affected?
• When Did Your Last Real Backup Actually Run?
• How Do You Fix File History Right Now?
• Should You Install KB5124010 or Wait for October 13?
• What Should You Do After the Fix Installs?
• Tips
• Warnings
• FAQ
• Sources

What Did KB5124008 Break in File History?

The September 8, 2026 Windows 11 security update KB5124008 (OS builds 26100.9445 and 26200.9445) caused File History to stop creating or updating backups on some PCs, and Microsoft acknowledged it as a known issue. According to BleepingComputer’s report on the fix, the failures affected backups and restores to external drives and network locations.

The symptom people noticed was a false “Reconnect your drive” alert. The backup drive was connected, visible in File Explorer, and healthy. File History just refused to use it.

An out-of-band update followed on September 14 (KB5129195), but reports from Windows Latest and Microsoft Q&A threads say it did not fix File History. The actual fix arrived eight days later.

How Do You Know If Your PC Is Affected?

A Windows 11 PC is likely affected if it runs build 26100.9445 or 26200.9445 (or a build between those and .9550) and uses File History with an external or network drive. Press Win + R, type winver, and read the build number in the window that opens.

Then look for any of these signs, which PCWorld and forum reports list as the common tells:

• The “Reconnect your drive” notification appears even though the drive is plugged in.
• Control Panel > File History shows a “Last backup” date that stopped moving in early September.
• Right-clicking a file and choosing Restore previous versions shows nothing recent.
• Event Viewer logs application errors for FileHistory.exe pointing at KERNELBASE.dll.

Table: Windows 11 24H2/25H2 update timeline for the File History bug, compiled from Microsoft Support KB pages and BleepingComputer coverage as of September 30, 2026.

When Did Your Last Real Backup Actually Run?

The quickest way to find the last File History backup that truly completed is to open the backup drive itself in File Explorer and look at the newest timestamps inside the FileHistory folder. Don’t trust the notification or the Control Panel summary alone.

1. Open the backup drive in File Explorer and go to FileHistory\[your user name]\[your PC name]\Data.
2. Drill into a folder you edit often, like Documents or Desktop.
3. Sort by Date modified. File History saves copies with the backup time in the file name, such as Report (2026_09_07 18_02_11 UTC).docx.
4. Write down the newest date you find. Anything you created or changed after that date has no backup copy.

When I checked the reports in the Microsoft Q&A threads, I noticed that most people who spotted the problem did so only when they tried to restore something. That’s the worst moment to find out.

If your newest copy is from September 7 or 8, your gap is the whole month. Note which projects changed since then. Those are the files to protect first.

How Do You Fix File History Right Now?

Installing the optional Windows 11 update KB5124010, released September 22, 2026, fixes the File History “Reconnect your drive” bug on versions 24H2 and 25H2. Microsoft’s KB5124010 page says the update “addresses an issue where File History might fail to back up or restore files to an external drive or network location.”

1. Open Settings and go to Windows Update > Advanced options > Optional updates.
2. Select the 2026-09 preview update listed as KB5124010 and choose Download & install.
3. Restart when asked, then run winver again. You should see build 26100.9550 or 26200.9550.
4. Re-select your backup drive. In Control Panel > File History > Select drive, pick the same drive again and save. Forum posts report this refreshes a stuck configuration.
5. Click “Run now” and wait for the Last backup time to update to today.

On Windows 11 26H1 the fix comes in a separate preview update, KB5124006, according to BleepingComputer. Same Settings path.

Can’t install optional updates on a work PC? Uninstalling KB5124008 also restores File History. It removes that month’s security fixes too, so treat it as a last resort.

Should You Install KB5124010 or Wait for October 13?

Most home users who rely on File History should install KB5124010 now rather than wait for the October 13, 2026 Patch Tuesday, because every day of waiting is another day of unbacked-up changes. The exception is if your PC depends on one of the two known issues Microsoft lists for this update.

When I compared the known-issues section of the KB5124010 page against the File History fix, two trade-offs stood out:

• USB Audio Class 1.0 devices — some older USB audio gear may fail to start with a Code 10 error in Device Manager. That means no sound and dead volume controls.
• Domain-joined PCs — some devices using Credential Guard may lose their trust relationship with Active Directory. This mostly matters for company laptops.

So here’s the call. A home PC with modern audio should install it. A home recording setup running an old USB interface? Back up manually to the drive first, then wait until October 13. A managed work laptop? Leave it to IT.

What Should You Do After the Fix Installs?

After installing KB5124010 or the October 13, 2026 cumulative update, confirm File History has taken a fresh backup and that you can actually restore a file from it. A green status alone isn’t proof.

• ✅ Control Panel > File History shows today’s date under Last backup.
• ✅ Right-click a file you edited this week, choose Restore previous versions, and see today’s copy listed.
• ✅ Restore one test file to a different folder and open it.
• ✅ Copy anything critical from your gap window to a second location, such as OneDrive or another drive.

Honestly, that last step is the one that matters. File History only keeps versions from when it runs. It can’t go back and save a file from September 15 that has changed since.

Tips

• Set File History to save copies every hour (Advanced settings) for a few days after the fix, then drop back to your usual interval.
• Keep one backup that isn’t File History, such as a cloud copy of Documents. A single tool failing silently is exactly what happened here.
• Put a calendar reminder for October 14 to check that the Patch Tuesday update installed and File History still runs.

Warnings

• Don’t reformat or replace your backup drive because of this message. The drive isn’t the problem, and wiping it deletes the history you still have.
• Don’t delete the FileHistory folder to “start fresh” before installing the fix — it won’t help and removes older versions.
• Uninstalling KB5124008 removes September’s security patches. Only do it if you truly can’t install the fix.

FAQ

Does this bug affect Windows Backup or OneDrive?

Microsoft’s known-issue description names File History specifically. OneDrive folder backup is a separate cloud feature, and reports recommended it as a stopgap while File History was broken.

Are my older File History backups still safe?

Backups made before the September 8 update stay on the drive. The bug stops new backups from being written; reports don’t describe it deleting existing versions.

Why didn’t Windows Update install the fix automatically?

KB5124010 is an optional preview update, so Windows only installs it if you choose it under Optional updates. The same fix reaches all PCs automatically with the October 13, 2026 Patch Tuesday release.

Sources

• Microsoft Support — September 22, 2026, KB5124010 (preview)
• Microsoft Support — September 8, 2026, KB5124008
• Microsoft Learn — Windows 11, version 24H2 known issues
• BleepingComputer — Microsoft fixes bug that broke Windows File History

Fact-checked based on Microsoft Support KB pages, Microsoft’s release health dashboard, and BleepingComputer reporting as of September 30, 2026. Build numbers and dates apply to Windows 11 24H2 and 25H2.