Microsoft has temporarily halted the rollout of its October 2018 update for Windows 10 (version 1809), writing in a page dedicated to the update that it would "investigate isolated reports of users missing some files after updating."
The update — a major revision to Windows 10 that includes tools like a cloud clipboard and a preliminary version of its phone mirroring software — was reported to have resulted in mass file deletion from user directories as well as less alarming issues like incorrect CPU usage in Task Manager or broken audio drivers, ZDNet reported.
One user on the Microsoft support board claimed to have lost 220GB of files. As CNET noted, some users reported that even using hard drive recovery software would not allow them to find more than a portion of the missing data.
Heads up to anyone updating windows. Apparently, if you have documents saved in your user directory, i.e. users/JohnDoe, and not one drive, the update will delete EVERYTHING in that location. So if your "Documents" or "Pictures" don't have a one drive symbol, MIGRATE IMMEDIATELY!
— Kevin Quintero (@coolKevinator) October 4, 2018
As far as I can tell, my computer downloaded the latest Windows 10 update around 3pm today and during that process it unlinked my Documents directory then unpacked its own setup files that overwrote my own files.
I am so angry at Microsoft right now - if this is the case.
— Windows 10 deleted my cat (@MakatiCub) October 4, 2018
Microsoft said it was investigating reports of data loss as early as Thursday, per Business Insider, though at the time it said that generally only advanced users would have installed the update before Windows begins sending automatic update prompts on October 9.
There's been speculation the issue is linked to OneDrive, Microsoft's online file hosting and synchronisation service, or somehow linked to a bug in user profile settings. According to Engadget, the bug appeared to have somehow slipped through early testing despite reports of the issue appearing on Microsoft's Feedback Hub months ago.
There are a lot of low upvote reports, and Feedback Hub auto-collection failed, so they got buried in the noise. Will be interesting to get a RCA on this from the @windowsinsider team and hear more about how they're addressing this tough problem.
— Rafael Rivera (@WithinRafael) October 6, 2018
However, the root cause of the file loss has not yet been determined. In the meantime, users who have already downloaded the update or are enrolled in programs like Windows Insider should not proceed with version 1809 until Microsoft has released a fix.
Users who wait for automated updates generally don't need to do anything, except make sure their machines haven't somehow downloaded the broken version yet, and maybe back up their files in case something goes terribly wrong.