Originally Posted by jamesewood
I save the MOBI files into a single directory, open Kindle, and then set the new Kindle content folder to be the newly created directory.
This worked for me using the latest Calibre (0.8.35) and K4PC (1.8.3 (36179)).
When you do this, are the MOBI files all the the specified folder (not in sub-folders) and do the existing ebooks from the original K4PC folder get copied to the new K4PC folder you specify? That is what worked for me. I only had 6 books on the device originally and I copied another 6.