I would dare being a bit bolder, this is how I did move folders on my own server:
- Make sure the folder is up to date with the remote. (probably not strictly necessary)
- Stop Syncthing (Pausing the folder could be enough)
- Copy the folder to the external drive (e.g. using rsync or equivalent).
- Edit the path in syncthing config file (config.xml)
- Restart syncthing
- Wait for scanning and initial sync (exchanging and comparing metadata to be sure the contents are the same as on remote) to finish.
- if everything works ok, delete the original folder
I don’t think the drive being external has any impact on syncthing unless presence is unreliable.