Works now. Very strange. I have no idea what triggered it. All the time I could access the Pi with the Mac's Finder or the app Pathfinder (that I normally use instead of the Finder app) via smb and could do everything with every file and folder except the mounted USB drive, because of insufficant rights (which was wrong). As Apple isn't very good with network I normally use cifs instead of smb. This somehow didn't work with Pathfinder. But as I could access the other parts of the Pi, I never thought this was the problem with the USB drive. So I now used cifs within the original Finder app which somehow worked fine with the USB drive on the Pi. Back to Pathfinder it then worked here too. And now the normal smb command works fine too. I'm confused. And scared.
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