I've reinstalled from scratch and everything is, as expected, just fine. It really is simplicity to start over, which I why have no qualms tinkering. Safe in the knowledge that nothing is irretrievable.
I've enough unix knowledge to navigate myself around, understand the basics and make adjustments accordingly - and no doubt enough knowledge to get myself into trouble! Still, that's how one (me at any rate) learns. I've reinstalled enough systems now (android, linux, windows) to know I can always recover a situation. I was once a programmer, and still dabble with scripts - albeit largely ripped off and repurposed but always understood. So I know errors happen, and I now they are triggered by something.
If an error message is insufficiently helpful, where in a former life I'd delve into code to find the reason, then I like to try and work out why it has occured. That helps my understanding of a system. There's a reason this 403 is thrown; the correct data is there so maybe it can't be read due to permissions, or found due to paths. Whatever, main thing is that I have a system up and running again.
Yes, I was trying to mount an external drive, a network share. That's nothing new to me and I appear to have got that sorted. However, on this new installation iteration I might use an attached USB stick as suggested elsewhere.
I've encountered another issue which I'll research a little and then maybe raise on a new thread. I may be under a mistaken belief over something, I need to rule that out first.