Ahhh Onedrive could be causing some issues there. Syncing folders and UE (or any game engine for that matter) have issues when binaries are being synced and things need accessing. Onedrive used to cause all sorts of issues especially if you sync files constantly between two rigs. If possible I’d recommend using some other form of source control, unless the nature of OD has changed since I had to fight binary problems.
That said, my projects are a different drive from UE but it’s not an external. I’ve seen issues with external drives not registering immediately, so when the engine checks it’s last known location for it, it fails and the list get’s cleared. Is your drive an external?