Syncing a folder on external drive on multiple machines?

I’m currently syncing media between a folder on my external drive connected to computer A, and my phone (device B).

If I install Syncthing on a third computer (device C), and connect the external drive and add the folder with the same folder ID and the other two device peers, will this setup work or will Syncthing get confused? As far as I can tell the .stfolder in the sync folder only tells Syncthing that the mount exists/does not exist and does not keep the index of the folder. Does each device keep a separate index?

For example, say I finish syncing A <-> B. I then disconnect the external drive from A, so A stops syncing changes. On my phone, I add new media to the synced folder, then connect the external drive to C. My phone (B) and the second computer (C) will sync the changes. What happens if I move the external drive back to A? At this point A will not know of the newly added media but will see that there are new files in the synced folder that is not shown in the index. Will A delete the newly added file? Or will the new file just get added to the index?

Sorry if this is a stupid question but I’m not sure exactly how Syncthing keeps track of changes and would like to avoid getting my media folder nuked. Thanks!

Edit: to clarify, the external drive only moves across the two computers, device A and C. On the phone (device B), the folder is stored locally and available at all times.

I think this should work fine. The indexes will be separate, of course. Disconnecting the drive from one computer should be the same if you simply paused the folder and did changes to it while still paused.

How about doing a simple test with a temporary folder and seeing what happens first?

As long as you only add files, I think it should work. When you start modifying stuff, potential for conflicts arises, but nothing serious like deleting files.

Great, thanks @tomasz86 and @acolomb!