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: