As it happens I found my missing files in C:/windows/users/yourusername
Not in Old. Not in Documents. Just here: Which
made me very happy because close to a terabyte of files were missing including my books, (the pdf files, anyway, Kindle was still available) my movies, tv shows, and old time radio files, AND all my music files...mp3 and FLAC. Bought and ripped. They had just
kind of wandered over here from This PC/Documents which was where they were before the update began
BarryG_691 thinks it has something to do with OneDrive and the new "Storage Sense" setting.
At least, he thinks that is why some people's files are disappearing.
He wrote this on another thread:
"I am wondering if this issue has anything to do with the addition of OneDrive to the Storage Sense utility (Settings->System->Storage) in the October 2018 Update, which will delete locally-stored files that haven't been opened for a set number of days. I am also wondering if those who lost files had Storage Sense turned on prior to the update, and those who did not lose any files had it turned off. Thinking even further, could it be that the update process is turning it on by default with a value for the number of days which is causing the utility to remove files?"
I think that makes sense since I don't have the OneDrive app installed at all and I didn't lose files and, while he has the app, he didn't have "Storage Sense" on and he didn't lose any either.
What does everyone else here think?