Hi. I already resolved the issue I was having with Syncthing so I don't need any assistance. I just want to post my solution here in case other people run into the same problem using the Syncthing addon in LE. The solution might be more obvious to intermediate or advanced users but I want to share it anyway.
PROBLEM: When adding a folder for an external HDD in the Syncthing web interface, it asks for the directory path to sync the folder to. If you didn't manually add the external HDD as a source in the file manager and simply typed the directory in the "Folder Path" field first, it will create a virtual folder at that path and sync to that instead of the actual HDD. Eventually the interface will tell you your running out of space and you'll have to investigate what's going on or free up space.
SOLUTION: Add the external HDD as a source in the file manager, type in the same directory path in the "Folder Path" field (eg /var/media/{HDD Name}/), and this problem won't happen.
Cheers.