The world now knows why last week’s Microsoft Windows 10 upgrade deleted unlucky users’ files: the software treated the default user directory as ripe for destruction, because it thought the files were elsewhere. The upgrade has since been pulled.

Last week, some unfortunate punters who hit the “download” button on the Windows 10 October 2018 Update received the gift of extra disk space, because they lost their \Documents directories.

On a support page issued on Tuesday, Microsoft’s sparse explanation of the bug is as follows: “An incorrect timing calculation may prematurely delete user profiles on devices subject to the ‘Delete user profiles older than a specified number of day’ group policy.”

October 9, 2018—KB4464330 (OS Build 17763.55)
https://support.microsoft.com/en-us/help/4464330/windows-10-update-kb4464330

If you normally use the Disk Cleanup tool in Windows 10 to remove temporary and unnecessary files, after installing the Windows 10 October 2018 Update you need to be more careful. This is because Microsoft has quietly add the “Downloads” folder as an additional location that files can be removed.

Microsoft cites problems with the latest update package deleting user files.