Microsoft had a big problem with the Windows 10 October 2018 (version 1809) update when it initially rolled out; the update deleted critical files for some users. Microsoft opted to pull the update to investigate and make changes needed to prevent file deletion. The update is set to start rolling out again with Windows Insiders being the first to get the revised version. Microsoft says that it pulled the version 1809 update across all channels including Windows Server 2019 and IoT equivalents and notes that situations such as this are why it rolls out feature updates slowly.
The version 1809 update was available only to those who manually clicked the "check for updates" button in Windows settings. Microsoft notes that reports of data loss with the update were few, claiming data loss happened in only one one-hundredth of one percent of version 1809 installs. Microsoft admits that any data loss is serious and that it investigated the issue and has identified and fixed all known issues in the update and conducted an internal validation. Microsoft is making its Microsoft Support and retail store customer service personnel available to help customers recover data lost in the flawed original update at no cost.
The revised Windows 10 October 2018 update is rolling out to the Windows Insider community, and the results of that rollout will be studied, including pouring through diagnostic data from Insiders and listening to their feedback before the update is released broadly. As for what exactly happened with the flawed initial update, Microsoft is offering more insight into that. It says that a "very small" number of users lost files, and the data loss occurred if Known Folder Redirection (KFR) was enabled previously, but the files were still in the original folder location rather than having been moved into the redirected location.
KFR is used by Windows to redirect known folders for Widows, such as Desktop, Documents, Pictures, Screenshots, Videos, Camera Roll and others, from the default location
There are several ways this could happen according to Microsoft, including directing a known folder to a different drive. Redirection of that nature was likely to happen if users installed a new drive in a PC and then moved known folders to that new drive. The user might also have configured one or more Known Folders to redirect to another folder on OneDrive. Windows users who used an early version of the OneDrive client, and used OneDrive settings to turn on the