https://cinelerra-gg.org/bugtracker/view.php?id=487

The "/" did the trick. Possibly I also was aware of it 4 years ago 😀 

On the other hand, clicking down as mentioned, does list the files without the ending "/"

What you did "should" have worked and you do not have to go down 1 directory at a time but do not put a file name on that line and you should put a ending / on the line.

Well, I was able to move to the portable disk by clicking down one directory at a time from / to

/run/media/terje/Seagate_4TB_back/video/HDV-M2T

What I tried first was to write or paste in the whole path at once in the Directory field and then enter, but didn't get the file listed then. I know from before the Cin file menu can be a bit tricky some places ........?

I don't know much about using USB's, but looking at your screenshot, it looks like on the top of the Load menu, you have the usual directory (Katalog) of home.  I think you need to change this to what you see on the df command, which is

"run/media/terje/"  and then you should see Seagate_4TB_back.

If I am wrong, let me know and I will ask GG.

Probably basics and just me, but I can't find this drive in CinGG with File>Open and Directory right menu?

See the attached screenshot below ....