I found a hidden photos library on my external harddrive where also my main photos library lives. It's called
.100s-Mediathek.photoslibrary. My mac is set to German, Mediathek translates to "photoslibrary". When I open the package, it contains some photos and videos from an import, along with other files just as a normal photo library. I'm on MacOS High Sierra.
What is it for, where does it come from and is it safe to delete it?