Okay, I'll give it a try again: the extended HD, as mentioned before, so inclusieve with the iPhoto library, and with Mac OS extended-Journaled is connected directly to the USB input.
Then I open Settings->Privacy & Security, 'full disk acces' push the + button, give in my password, and push change settings.
Adding at the left side the HD (named 'Naamloos') but then, in the middle column I get to see the content of that HD, but many things are gray, including 'iPhoto-library #12'
Yes it is last opened with Mojave, as far as I can see.
If I try to open the same HD with my old MacPro 5.1 I get the message " you have opened this before, therefore it is not possible to open it now"
I hope my explanation is clear enough because of the language used (dutch to english)