@zing Thank you very much. I've seen this tutorial before, it doesn't address my problem. But you helped me anyway: I once again went to the command-line, and I asked myself: where am I actually? So, out of nowhere, I just typed:cd ..
Then cd .. again, and now I'm finally where I wanted to be all along. It was just unfamiliar to me: In Debian, being root and typing cd (without the two dots), brings me to the very "root" in Debian (shortcut). But in Recalbox it doesn't. It's probably due to the different partition, I'm not used to that. So: problem solved, much learned! Now I can finally access the share partition/folder how I wanted to.