Hi Remoc
My D drive is the one with “my documents” that type of thing.
It’s not an external drive. I’m afraid my external one broke, after just one year!
Avoid buying Seagate. They come with those screws with the stud sticking out of them so it really is a task to get them open.

Back on the subject of the D drive, mine is the permanent one. Basically even if I were to re-start my computer from scratch (like after a virus or something) it’s the drive that retains all the files.