To answer the questions first:
It automounted first time around, but I wanted to get rid of the NTFS system.
You are right, I should have written down the error messages, will do that next time.
I've got it up and running.
Apparently, the combination of
sudo mkdir /media/external
sudo cd media/external/
made it visible in Storage Device Manager
In the media/external directory it was visible as a (imagine: 1.4TiB !) floppy.
This time around, it showed up in GParted and formatting went OK.
After that, taking control of the thing with
sudo chown -R paul.paul /media/90d3ecb5-ba.............
did the trick. So now, I can move data to it.
Looks like problem solved.