Grrrrr. I give up! Just tried a fresh install of Mojave on a USB3.0 drive (again) with only the driver installed and it is now seen by the MBox Pro Preference app.
I'm now thinking that it's a conflict between the HD Driver and Mbox Pro software.
Alas, can't find out how to uninstall the HD Driver as it installs something on the PreBoot partition, so it's not just a case of removing DigiDal.kext from the hard drive as mentioned in other areas of the DUC
Any ideas, before I find it on Google. I'll post here if i find it. Don't all shout at once now!!!!