Hi Clivemcl and welcome to SF
Im not too wise on this subject, but i do know:
USB 3.0 front bays need USB3.0 connectors although USB 3.0 is backwards compatible.
Your existing USB 2.0 gear will work on version 3.0 ports and vice versa.
But plugging a 3.0 device into a 2.0 port or a 2.0 device into 3.0 a port will get you
standard USB 2.0 data rates...approx 480 Mbit/s.
Hope this helps somewhat.