Windows 10 v1809 October 2018 Update was hit by the worst bug till now. A lot of users who upgraded to it reported a loss of files from their user folder. It turned out to be a bug in Windows 10 Upgrade deleted those files because of what is Microsoft calling Known Folder Redirection (KFR) redirection issue. Microsoft is now rolling out a fixed version with ISO and Media Creation offerings, while those who have already updated to v1809 are getting a Cumulative Update 17763.55 (KB4464330) as part of the fix. Now let’s deep dive into what caused the data loss problem for those who installed Windows 10 v1809.

What caused the Data Loss in Windows 10 v1809

Known Folder Redirection is a technique which allows one to change the default location of folders like Downloads, Music, Documents, OneDrive, etc. to somewhere else on the PC without changing where they appear on the Windows Explorer.

When v1809 was under testing, a couple of users reported that when this technique was used, they had an extra copy of empty folder at those default location. Microsoft added a new set of code in the October 2018 Update to remove these empty, duplicate known folders. This changed combined with another change to the update sequence, resulted in the deletion of the original “old” folder locations and their content, leaving only the new “active” folder intact.

What is even surprising to note is that this was a remote issue which was reported by a set of users while this version was with Insiders. Since Feedback Hub is based out of voting, this major issue was buried as it did not get many votes from all the users. Shared by @WithinRafael on Twitter, it’s pretty clear that Microsoft had it overlooked. IMO any issues which are around file deletion must be taken seriously.

Windows Team adds further that the user files were deleted in the following situations:

1. If the contents of c:\users\username\documents were not moved to D:\documents when the location was being changed,  then a user could also encounter this issue.
2. When the user chose to redirect their folders to another folder on OneDrive but chose not to move existing files to new location. Since those files were not moved, October Update treated them as duplicate folders and deleted them.
3. If Known Folder redirection was used using an early version of the OneDrive client with settings to turn on the Autosave feature. The client didn’t move the old files to the new location.

So if the files were left in the old location they where removed by the Update. Now here is what I am surprised at. Why did users choose to leave the files after migration to a new location? Secondly, why were they looking at these folders when they had already migrated to a new location?

On a related note – comments like these should cause concern to Microsoft:

The Win 10 updates are unpredictable. I’m scared to death each time the automatic updates (which I can not disable) happen. After an update one of more installed software stop working and need re-installing. I have no viruses or cracked software. The last Win 10 update corrupted the MBR (Master Boot Record) on my 2 laptops and 1 desktop. 3 computers done the same update at the same time and having corrupted MBRs is NOT a coincidence. Microsoft owes $900 just for this damage alone. i spent $300 for each computer to have the drives removed and put new drives in so I can keep my data that was not corrupted in the old hard drives. The “reset Win 10” without losing personal data function in the advanced settings was not working on all 3 computers. Thank God I had 3 spare SSDs as a backup, but it is such a hassle going through the old hard drives and saving all the data in different locations. I’m switching to MAC and ditching everything PC related – PERIOD!

If you lost your data files, you can try and recover those files.

Windows 10 v1809 October 2018 Update is now available for download again. If you wish to still play safe, you may defer the installation of this feature update.