Just found a Windows.old folder dated 9/21 with 17gb of data.  This may be the revert folder from the anniversary install?  

I see that too. Wondered why my C drive was filling up...

You can delete that folder using the windows disk cleanup tool if you do not plan on going back from windows 10.

biodiode
You can delete that folder using the windows disk cleanup tool if you do not plan on going back from windows 10.

disk cleanup is a nice utility to get rid of crash dumps, installer remnants etc.
 
since win10 they spilt into the GUI into two halves i.e. user files and win 10 ("clean up system files") ... the later you should use only once you are sure you dont want roll back to the OS before the previous win update (or if you have an external backup)

The contents of Windows.old are normally removed automatically after 30 days.
 
AMB
 

https://windirstat.net/