So, the problem can be summarised as follows:

Login using ‘graphical boot’ (runlevel 5) and KDM – everything works correctly.

Login using ‘multi-user boot’ (runlevel 3) and then use # startx to run KDE – USB disk mounts fail with “… Not Authorized”

I use a non-graphical login (runlevel 3) and then run # startx, as this is most convenient for my particular needs (and Linux is all about choices, correct?)   I have been using this login method for – approximately – the last 15 years or so…    So – I have applied the hack/workaround as detailed in my previous post..

After some confusion as to the actual cause, this is now an active bug at Redhat/Fedora Bugzilla:

https://bugzilla.redhat.com/show_bug.cgi?id=820675