I was able to get this.
You need to close the plex app. Move whatever you he in “%LOCALAPPDATA%\Plex\Plex Media Server\Sync” to the bigger drive. example: D:\Plex\Sync Once the transfer is complete. You need to create a link. The command for that link (need admin level command prompt).
Command: mklink /J “D:\Plex\Sync” “C:\Users\YOURUSERNAMEHERE\appdata\local\plex\plex media server\Sync”
This will create the symbolic link in the appdata folder, it will look like a shortcut. Make sure the folder Plex\Sync exists on your other drive, then open the symbolic link in your appdata to test and make sure it works. If it works then you are pretty much all set! If you had to move existing offline items in the first step, make sure they are under the new folder in the correct directory format, should look like D:\Plex\Sync\1 etc.
These instructions assume you know what your user name is, and the secondary disk is using D for the drive letter. So please change the items here accordingly to match YOUR setup.
Hope this helps!