USB Mass Storage not working

I just got the Fenix 5X Plus (titanium) and it refuses to mount as Mass Storage Device.
I have tried with two Macs and two cables.
The 5X that I still have is working with both Macs and both cables.

Garmin Express will recognise the 5X Plus and transfer updates, music, CIQ stuff, etc. just fine with good speed.
But no drive mounted on the computer for manual transfer of beta, maps, etc.
Both computers can see the device "composite" in the System Information app but none of them will see it in Disk Manager (or whatever it's called on English systems.

Any clues, solutions?