Well, let’s not get ahead of ourselves, here.
Firstly, I’ve restarted the computer, which is something I very rarely do. But, you know, the old IT maxim “have you tried turning it off & on again?” & all that.
I’ve run Disk Utility & that showed no errors on the external hard drive. Although I do accept that it’s not an SSD so that comes with its own problems.
My Roon is set to back-up daily, although the back-ups appear to be a load of random folders. Nothing useful to a layman like me, such as a file with a time & a date on it. But anyway, the back ups are there.
Before I do anything else, I’d really like some clarity on a some of my questions…
When it’s doing this “applying library settings” thing & I get fed up with the impaired performance & restart Roon, it doesn’t continue to do it after the restart. This is different to the “analysing files” thing, which immediately picks up where it left off. Could it be that the restart clears the “blockage” & it completes the “applying” process quickly, without me realising it?
How would I know? Is there somewhere that it keeps a log of applied library settings?
My system is set to back up at 3am every morning & I know that it does this because I’m often up at that time & have to stop & wait while it does it. Why does it seem to randomly & occasionally decide to embark on the “applying library settings” mission, independently of my back-up preferences?
Is there a note of the last successful back-up anywhere?
What harm will it do (if any) if I don’t let it finish “applying” but instead restart Roon?
Why does it not seem to pick up where it left off after the restart, the way “analysing” does?
Also, in almost every other process that Roon does, there is some kind of visual feedback. “Analysing” tells me how many files of how many, as it goes along. When I launch Roon it shows me the number of scanned files as it goes. When I add music it tells me how many have been added & how many have been identified, etc.
“Applying library settings” seems to be the only process that keeps me completely in the dark while it’s doing whatever it’s doing.
Incidentally, I still don’t know what it’s actually doing.
I really appreciate the level of help I’m getting here, by the way. I still firmly believe that most of these issues are cause by Dropbox & Roon fighting for bandwidth because they’re both so greedy. Although they’re not in any way connected, they both seem to operate on the assumption that, in any given household, a mi casa su casa principle is in operation whereas in reality there is a whole family who need to share the facilities! 
