F5 running FW5.40 not recognised on PC

I always had much patience with my Fenix products (F3, F3OHR and now F5) and have been patiently following the Beta process... (I use a stage PM )

Now finally I see a beta version that should improve the ANT+ connection, so I get ready to install it...

Oh joy, my F5 running the 5.40 firmware refuses to show up as USB storage, neither is visible in Garmin Express???
Anybody had the same issue? Any known solutions before I throw the thing out of my window?

thanks in advance!