Thanks, yes of course. I got excited about fixing my problems and reported too early 🙂
Well, the VM has been working fine since the export / import. I can see both drives.
Although the VM running fine, since the last post I had to address two related VDIs that had been connected to the Control Domain. One being million_D and the other million_D (with a camera icon). These probably ended up here because of a aborted / faulty snapshot or backup. I disconnected them .. checked a few things ... and then forgot them. Everything is running fine now.
For future googlers: During the problem solving, I found out some pretty helpful info using tail -f /var/log/SMlog.
To me, this issue is solved. Thank you all for the feedback.