I have external USB3 backup disks that auto-mounts on Leap 15.2 when connected, and is listed in both Gnome Files and terminal. i.e drive /dev/sdb1 mounted on /run/media/terje/Seagate_4TB_back
Probably basics and just me, but I cant find this drive in CinGG with File>Open and Directory right menu?
See the attached screenshot below …
File-Open_from_Portable_drives.png
I dont know much about using USBs, 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.
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 didnt get the file listed then. I know from before the Cin file menu can be a bit tricky some places …?
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.
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 /
I did report a feature adjust suggestion regarding File listing on Mantis BT: