Update: According to The Verge, Microsoft has pulled the October Windows 10 update until this behavior can be addressed.
Be warned: users who’ve tried installing early previews of the next big Windows version update, version 1809, have found it has deleted their personal files. Some users have lost hundreds of gigabytes of personal documents, photos, and other material. This behavior is apparently tied to updates to the Windows space-clearing tool—it gets a little overzealous when it comes to clearing out old user profiles. Have a care before you update—if you keep your ebooks in your documents folder, you could end up losing them too!
Fortunately, there are precautions you can take, even beyond the common-sense approach of backing up your files externally. Perhaps the simplest precaution is to disable Windows’s automatic updates until a fix can be found. But for people a little more tech-savvy, there’s also a workaround involving using Windows’s Group Policy Editor, gpedit.msc, to disable a key setting.
The Group Policy Editor is bundled with Windows 10 Pro versions, but Windows 10 Home users can add it easily. Once you have it installed, run it and navigate to Computer Configuration/Administrative Templates/System/User Profiles/Delete user profiles older than a specified number of days on system restart. Right-click this property and choose “Edit,” then set it to DISABLED. I’ve tried installing the editor and changing the setting myself, and it seems to work.
Changing that setting should defang the update and let you keep your files—but just in case, you should probably still back them up to somewhere else anyway.
Special thanks to Mercedes Lackey for pointing the problem and solution out to me.
On a related note, the new Windows 10 update’s space-clearing tool can also empty out your downloads folder if you’re not careful. So, best be careful!