This sdd was an internal drive before I change the computer to a nuc form-factor, so now it sits in a box attached by a cable. I don't know what docker is, too many confusing explanations to follow.
Anyway, /Disk with the padlock has disappeared and a new folder which I created when I moved multiple GB in folder to the SDD to clear up space on my internal drive. There's also a new folder called /.trash-500, not really sure what created that one.
Oops, sorry got sidetracked with someones book, thanks
Re: "too many confusing explanations to follow."
* Most of us have our own methods of investigating issues. I usually check the kernel messages on a fresh boot, reproduce the issue, then look at them again. Some prefer systemd's journalctl as is presents info on additional system components beyond basic kernel stuff.
* Docker is (in this context), it's a containerization software (and full-on platform to split hairs):
https://www.docker.com/
(don't worry about it).
@dos2unix likely was trying to eliminate the possibility of your running PCLinuxOS in a container, as that would've given some useful insight if you had been.
* The link to permissions posted by
@Jarret B was a useful tutorial for not just managing, but understanding permissions as this appeared to be permission-related (well, is still technically, except now your system is managing it properly).
Off-topic: That article is very comprehensive and definitely worth bookmarking. It explains things better than I, or anyone I know, ever could.
Re: Your new questions
* The .trash-<number> is part of the "trashcan" feature. It may safely be deleted if you don't use the trashcan feature.
* Whatever that /Disk directory was, it was obviously a temporary mountpoint. I have no idea how/why the PCLinuxOS folk did it this way, but hey, it's an open source world.
Closing commentary: So far as how things are seeming to "fix" themselves, I'm guessing it's because you're using a "managed system" (if I can apply that term in this context) and not handling things manually. So something went wrong somewhere and "fixed itself" kinda like "did you turn it off and on again". That's the only up side of managed systems, though for me there's nothing more tedious than tracking down what's to blame for a problem in a managed system, so I try to strike a balance, sort of I manage what causes most issues and leave be the rest...
But I digress, so long as everything's working for you, that's great as it gives you time to gradually build up the knowledge to troubleshoot, and although learning is really a lifelong process, it's a good one since you have a real feeling of accomplishment each step of the way.