Hello Hansmartin,
I assume you've told macOS to launch Hauptwerk automatically when macOS boots (i.e. automatically upon log-in).
I think your diagnosis is correct in that macOS is sometimes launching Hauptwerk before macOS has mounted your external SSD, resulting in Hauptwerk not being able to see the files on that SSD.
There's no way within Hauptwerk itself to make the Hauptwerk executable wait upon launch before trying to access any of its files, I'm afraid. (The 'General preferences | Advanced ... | Minimum time to wait on start-up' setting behaves somewhat like that, to give the operating system time to finish loading audio/MIDI interfaces' drivers, but that delay happens after Hauptwerk has read its file structure [not least since user settings, such as that one, are saved within files], so it wouldn't help you in this case.)
You would potentially need to come up with some other means to launch Hauptwerk (instead of macOS' standard right-click option to run it automatically upon log-in). E.g. you could probably write a shell script that implements a delay and then launches the Hauptwerk executable, and then make macOS launch your shell script, instead of launching Hauptwerk directly.