Had another major update on my machine this week, which I kept putting off as too busy to be looking at blue screens again if it went a bit silly (again).
So let it run, and sure enough update took awhile, with lots of, 'its improved, new features, dont turn off your PC' blah blah, with a message at the end saying allk your data is where it was or something on long them lines, anyway, when finished, I noticed I was 25GB better off on my C drive, which you may or may not consider it be advantages, but 25 GB of free space is a fair chunk, especially when I go the the trouble of storing most of my data on other partitions of the HD to keep my C drive low and to make back ups easier.
So my intial thoughts were the update has probably emptied my download folder and or any restore points data, but no, it had actually wiped my Windows Old / Users Data, things like App Data, Desktop Folders etc, the Folders where still there but not much in them.
The windows old, is or was, in my case, pretty much a windows image or your old OS system, so should you decide to roll back to your previous OS, this was the important bit, but it also contains any data or files you had kicking around that
didn't form part of your Local Folders, like documents, Pictures, Images, Downloads etc.
So if like me you had files and folders on your old destop or you need access to old App Data files, then
back them up now, or just take a System Image of Windows Old and stash it some where, taking a image of it also lets recover your old system when used system recovery boot method as a last resort. Win 10 is also pretty good at managing disk image files easily.
Luckerly for me I did a back up of Windows / Old when I moved over to Win10 both before and after the Install.