I’m going to chime in here. I’m able to fully sync (read & write) folders on my external SD card in Android 7.0 without root and WITHOUT formatting my SD card as adoptable storage (I kept it in it’s removable exfat format). Tested on my Moto X Pure running stock Android 7.0 and on my LG-G3 running Lineage 14.1 Android 7.1.2. In the Android app under Settings/Behavior there is an option to use the Advanced Folder Picker, check this off. My external sd card is labeled /storage/0123-4567 on my device. This number will vary depending on your phone. In this particular setup I’ll explain how I sync my entire 90gb music library to my external 128gb sd card on Stock Android 7.0, MotoXPure:
1: Add your computer to Android by scanning it’s ID. Wait for it to add, and don’t select any folders to share with it.
2: On your Android, create the folder you want to sync with and create a .stfolder inside the folder (or just add a .stfolder to an existing folder if you already have the folder created the device).
3: On the computer create a new folder share within Syncthing’s GUI. In my case it was my Music folder on my PC. Select the device to share the folder with (your Android) and choose all of your preferences. I turn all file versioning off for my needs and on my PC I was sure to set it as “Send Only” so that I can’t screw up and delete something from my PC’s master music folder.
4: Wait for the notification to appear on your Android, then click it when it arrives. Name the sync to whatever you want and then in the “Path” area select your external sd (which will be labeled something like /storage/0000-0000). Then once the SD is selected, navigate to the folder on the external sd that you’ll be syncing (in my case it’s /storage/0123-4567/Music).
5: Now wait for the devices to re-communicate with each other and syncing will begin. You may need to restart Syncthing on both devices to get it to initiate.
6: That’s it…both folders are now syncing. Any music file I add from my PC will write to my phone’s external SD if Syncthing is running. Even though I am rooted, root is not required.
Note: Both of my folders (music on PC and music on Android’s external sd) were both already on each device. After setting up the sync, the initial scan took quite a while to complete (about 40min), but after that it’s working properly. All changes are synced immediately based on your selected scan interval. I took one album and changed the tags & embedded a cover image into each mp3 file and they were synced to my Android almost immediately. Can’t be happier, this is working great.