Windows 10 October 2018 Update hit the web last week but was pulled after a couple of days due to a serious file deletion bug. Dozens of reports hit the web from users who had lost their entire Documents folder and now we know why. Microsoft explains the bug affected people who had used Known Folder Direction, a feature that moves new files to another disk with free space. The bug in Windows 10 October 2018 Update removed the original folder locations and their content, leaving only the new active folder intact, without checking if the old folder location were empty.
The Register lists three possible file deletion scenarios below:
A user redirected KFR to a different drive, but only put new files there, leaving their old files in the default user directory. Files in the “old” folder were deleted;
The user created a redirection from one folder to another, but again, used that location only for new files, leaving the old files in the default (Cable noted that during KFR setup, users are prompted to decide whether or not to move the files); or
Users of older OneDrive clients, who had enabled auto-save. Again, this is because there's a redirection, but old files remain in their default location, because OneDrive only used the redirect for autosave (for example, in a c:usersusernameonedrivepictures folder).
As the site points out, the fiasco raises new questions about Microsoft's quality assurance. Especially as the bug was pointed out by testers of Windows Insiders build on at least two separate occasions.
Microsoft is pushing the fixed Windows 10 October 2018 Update build to Windows Insiders and will do a mainstream release once it confirms there are no other issues.