Windows 11's mandatory September security update, KB5124008, shipped on September 8, 2026 and moved 25H2 to build 26200.9445 and 24H2 to build 26100.9445. Buried in Microsoft's own changelog for that update, listed as a known issue, is this: some customers using File History — the built-in tool that backs up your files to an external drive or a network location — might be unable to create or update backups after installing it. The update that was supposed to patch security holes quietly broke the thing meant to protect your files if something else goes wrong.
What actually happens
Microsoft's own description says affected devices can show an incorrect "reconnect your drive" message even when the backup drive is properly connected, and that the "last backup" timestamp stops updating — so a backup that quietly stopped running still shows a date that looks recent enough not to worry about. Some users also reported seeing "no previous version available" when trying to restore a file that had, in fact, been backed up before the update installed. None of this throws a hard error. That's the part worth sitting with: File History doesn't tell you it's broken, it just stops working while looking like it hasn't.
Which machines have it, and what Microsoft has done so far
Microsoft's changelog for KB5124008 doesn't say the bug is universal, but coverage of the issue describes it as affecting most machines that installed the September update and use File History — both the 24H2 and 25H2 builds. An out-of-band fix on September 14, KB5129195, didn't resolve it. A second attempt, KB5124010, shipped September 22 as an optional preview update — not one Windows installs automatically the way it does a security patch — bringing builds to 26200.9550 and 26100.9550. Microsoft's own changelog marks the File History issue "fixed" under that update, but outside reporting from Windows Latest found it wasn't resolving the problem for everyone who has it, as of September 23. Whether "fixed" holds up for your specific machine is something you have to check yourself, not something the changelog can promise on its own.
How to check if you're affected
Open Settings, System, About and look at the build number. 26200.9445 or 26100.9445 means you installed the September 8 update without the September 22 preview fix. If you've since installed KB5124010, you'll see 26200.9550 or 26100.9550 instead — worth checking, but not a guarantee the underlying problem is gone for your setup. Either way, the check that actually matters is in Control Panel, System and Security, File History: look at whether the "last backed up" date is genuinely recent, not just present, and try restoring a file you know changed recently to confirm the backup behind it is real.
What to do in the meantime
If you depend on File History and haven't installed the September 22 preview update, Settings, Windows Update, Optional updates is where to find it — it won't arrive on its own the way the original security patch did. If you have installed it and still see a stale timestamp, the safest move for anything you can't afford to lose is a manual copy to a second drive or cloud folder until Microsoft confirms the fix is complete, rather than trusting the tool that's had two attempts at a fix in two weeks. If you're one of the machines still running Windows 10 under Microsoft's extended free security updates, this particular bug doesn't reach you — Windows 10 isn't on the affected build numbers — though that's a fact about this bug, not a reason to avoid the eventual move to 11.
This is the third company in a month to ship a fix that broke something more basic than what it was patching: some Pixel phones stopped unlocking after their own September update, and Apple confirmed a separate reboot bug on the iPhone 18 Pro days before that. None of these say anything about the hardware underneath — they're all bugs introduced by patches meant to fix other things — but they're also a reason to verify a backup with a real restore test now and then, instead of trusting a timestamp that can lie.
Last updated September 27, 2026