Here we go – Ride the wild software time.
On Windows
- Turn off all automatic updating
- Disable “Empty Trash”
- Save those settings
- Stop Plex
 (we’re about to clone it)
- Create a ZIP file of the “Plex Media Server” folder (the one that contains all the metadata underneath it)
On DSM
- Create a shared folder named Plex(case sensitive)
- In that shared folder, create this structure (old DSM 6 'cuz it’s easiest)
Plex/
  Library/
    Application Support/
- 
Copy the ZIP file to the Plex shared folder 
- 
Unzip the ZIP 
- 
After unzipping move the contents if needed to product the following structure 
Plex/
  Library/
    Application Support/
      Plex Media Server     <-- this will have come from Windows machine
- 
Download PlexMediaServer from https://plex.tv/downloads for your Syno 
- 
Manual install that SPK file 
- 
As it installs, it will find the “Plex” shared folder and immediately (while installing) start migrating it to the DSM 7 structure. 
- 
Depending on how much data you have, you might be here a few minutes → hours → whenever. 
- 
Open a new window to the Synology, Open the Plex share. Look at the file “Migration.log” . You’ll be able to follow the progress 
In case you need to see the steps about granting permissions.