On September 22, 2026, Microsoft released KB5124010 and KB5124006 to fix Windows 11 File History. Both are optional preview updates. On Windows 11 24H2 and 25H2, the trouble started with the September 8 security update KB5124008, which left File History showing “Reconnect your drive” while it quietly stopped backing up files. Microsoft lists the same known issue on its September update pages for other Windows versions too, and as of September 26 it still lists it as unresolved on Windows 11 23H2 and on Windows 10 21H2/22H2, 1809, and 1607.
What Happened
Microsoft’s own documentation lays out the timeline. Every date below comes from Microsoft’s support pages or from ghacks.net’s September 25 report.
| Date (2026) | Event | Source |
|---|---|---|
| September 8 | KB5124008 security update ships for Windows 11 24H2 and 25H2 | Microsoft Support: KB5124008 |
| September 19 | Microsoft adds the File History failure to the KB5124008 known issues | Microsoft Support: KB5124008 |
| September 22 | Optional preview updates KB5124010 (24H2/25H2) and KB5124006 (26H1) ship with the fix | KB5124010, KB5124006, KB5129195 |
| September 26 | Windows 11 23H2 and Windows 10 21H2/22H2, 1809, and 1607 still listed as unresolved | KB5129242, KB5129236, KB5129238, KB5129239 |
What broke
Microsoft documented three symptoms on affected PCs:
- File History showed “Reconnect your drive” even though the backup drive was plugged in and working.
- The Last Backup timestamp stopped updating.
- The Restore personal files screen showed “No previous version available” when you tried to recover a file.
Behind the scenes, Event Viewer logged crashes tied to FileHistory.exe and KERNELBASE.dll. That’s the key detail for help desk readers. The backup engine itself was crashing, along with the status display.
Why this bug was so easy to miss
File History runs in the background. Most people set it up once and never open it again. So a failure that doesn’t pop up a loud error is a failure nobody sees.
The “Reconnect your drive” message made it worse. If your drive was plugged in, the message looked like a glitch you could ignore. Meanwhile, new and changed files weren’t being copied. Based on Microsoft’s dates, some PCs could have gone about two weeks without a real backup (September 8 to September 22). Nobody would know until they tried to restore something and got “No previous version available.”
Who this affects
- Windows 11 24H2 and 25H2: Affected by KB5124008. Fixed by the optional preview update KB5124010 (builds 26100.9550 and 26200.9550).
- Windows 11 26H1: Fixed by the optional preview update KB5124006.
- Windows 10 21H2/22H2: Microsoft’s KB5129236 page still lists the File History problem as unresolved.
- Windows 11 23H2: Microsoft’s KB5129242 page still lists it as unresolved.
- Windows 10 1809 and 1607: Microsoft’s KB5129238 (1809) and KB5129239 (1607) pages list the same issue, also unresolved. Treat them as not covered until Microsoft names a fix.
If you don’t use File History at all, this bug doesn’t touch your files. It only matters if File History backs up to an external hard drive or network share.
Did it delete your existing backups?
Microsoft’s documentation doesn’t say that existing backup data was deleted. The documented symptoms are backups that stopped updating and a restore screen that couldn’t show earlier versions. The safest reading is that new backups stopped. Whether older versions were still readable during the bug isn’t spelled out.
Either way, don’t format, wipe, or repurpose your backup drive while you sort this out.
How It Stacks Up
Fixed vs. not fixed, by Windows version
| Windows version | Fixing update | Status as of Sept 26, 2026 | Source |
|---|---|---|---|
| Windows 11 25H2 | KB5124010 (build 26200.9550) | Fixed in optional preview update | Microsoft Support |
| Windows 11 24H2 | KB5124010 (build 26100.9550) | Fixed in optional preview update | Microsoft Support |
| Windows 11 26H1 | KB5124006 | Fixed in optional preview update | Microsoft Support |
| Windows 11 23H2 | None named | Unresolved, Microsoft working on a fix | Microsoft Support (KB5129242) |
| Windows 10 21H2/22H2 | None named | Unresolved, Microsoft working on a fix | Microsoft Support (KB5129236) |
| Windows 10 1809 | None named | Unresolved, Microsoft working on a fix | Microsoft Support (KB5129238) |
| Windows 10 1607 | None named | Unresolved, Microsoft working on a fix | Microsoft Support (KB5129239) |
Two weeks from bug to fix is quick for the newest Windows 11 branches. The gap is on older versions. Those are exactly the machines that home offices and family PCs tend to hang on to.
Should you switch backup tools?
Microsoft hasn’t told anyone to stop using File History. It fixed the bug. Here’s how the built-in options compare, based only on what the research supports:
| Option | What it does | Replaces File History here? |
|---|---|---|
| File History | Automatic, versioned copies to an external drive or network location | The tool that broke; now fixed on 24H2/25H2/26H1 |
| Windows Backup | Moves settings, apps, and supported files to a new PC or restores after a reset | Not established as a like-for-like replacement for versioned backups |
| OneDrive folder backup | Cloud sync for Desktop, Documents, and Pictures | Not a documented substitute, but a common second layer |
The takeaway: nothing in Microsoft’s documentation argues for ripping out File History. A second backup layer, like OneDrive folder backup or a copy to a separate portable SSD, would have caught this gap.
The Reaction
Coverage has been mostly informational. Windows Latest (September 23) focused on the false “Reconnect your drive” alert appearing on drives that work fine everywhere else. It said File History works again after KB5124010 but is “not fully patched up for everyone,” citing a reader whose Run now still didn’t update the Last Backup time. PCWorld (September 23) stressed that the fix is optional and won’t install on its own; otherwise it arrives with October’s Patch Tuesday update. PCWorld also mentioned uninstalling the September security update as a stopgap. BleepingComputer covered both the breakage and the fix.
That “it’s fixed” framing is part of the problem. Microsoft’s own pages still list Windows 11 23H2 and Windows 10 21H2/22H2, 1809, and 1607 as unresolved. If you skim headlines, you could walk away thinking your Windows 10 PC is safe when it isn’t. The quiet reaction fits a quiet bug: people don’t get angry about backups they don’t know failed.
Our Take
Verdict: Install the fix now. Don’t pause updates, and don’t roll back KB5124008.
If you’re on Windows 11 24H2, 25H2, or 26H1, the fix is available and Microsoft documents it. Because it’s an optional preview update, Windows won’t install it for you, so install it manually. Pausing updates keeps you on the broken build longer. Rolling back KB5124008 isn’t a remedy Microsoft lists, and it would strip out that month’s security patches. Microsoft doesn’t describe a manual workaround either. Microsoft says the issue is resolved in Windows updates released on and after September 22, 2026, so later monthly updates for these versions will carry the fix too. The September 22 preview is simply the way to get it now instead of waiting.
How worried should you be? Moderately, and only for a specific window. The risk isn’t lost old backups, since nothing in Microsoft’s documentation says those were erased. The risk is that files you created or changed between September 8 and your fix date may never have been backed up. If your drive failed or a file got overwritten during that stretch, File History may have nothing to give you.
On Windows 10 (any version) or Windows 11 23H2, be more cautious. Treat File History as unreliable until Microsoft names a fixing KB for your exact version.
What to do next
On Windows 11 24H2, 25H2, or 26H1, here’s the one practical thing to do: install the fix, then confirm File History actually ran. (On Windows 10 or Windows 11 23H2 there’s no fix to install yet, so skip to the note after these steps.)
- Open Settings > Windows Update and click Check for updates.
- Install KB5124010 (24H2/25H2) or KB5124006 (26H1) when Windows Update lists it. It’s optional, so it won’t install automatically.
- Restart your PC.
- Press
Win + R, typewinver, and press Enter. On 24H2 you want build 26100.9550 or later. On 25H2, 26200.9550 or later. - Open Control Panel > System and Security > File History.
- Check that the Last Backup time shows today, not a date around September 8.
- Click Run now if it’s stale, then check the timestamp again.
- Click Restore personal files and browse a folder you use daily. Make sure recent versions appear.
You can also confirm the update installed from PowerShell with Get-HotFix -Id KB5124010 (swap in KB5124006 on 26H1). If it’s installed, PowerShell lists that KB. If the command returns an error instead, rely on the winver build number from step 4, which is the same build number Microsoft publishes for the fix. For deeper detail, run eventvwr.msc, go to Windows Logs > Application, and look for application errors naming FileHistory.exe with KERNELBASE.dll. If those crashes keep appearing after the fix installed, check the backup drive itself, and remember that Windows Latest reported the fix isn’t complete for everyone.
If you’re on Windows 10 or Windows 11 23H2: Keep installing updates as they arrive, but don’t rely on File History alone. Copy your important folders to a second external drive by hand, or turn on OneDrive folder backup for Desktop, Documents, and Pictures. Keep in mind that OneDrive syncs those folders to the cloud rather than keeping versioned backups on a drive, so treat it as a second copy, not a File History replacement. Check your version’s Microsoft Support page for a named fixing KB.
Wrapping Up
This bug didn’t wipe anyone’s backups, as far as Microsoft has documented. It did something sneakier: it let File History look configured while it quietly stopped doing its job. Whatever version you run, the Last Backup date is the one number worth checking this week.