Follow up...
I *think* that I have managed to get everything installed correctly but I installed bootmenu with the external card removed from the device so I don't actually get a boot menu at startup (even if I set the default startup to the cloned partition on the external card after re-inserting it.

Problem is, the only way to get the boot-menu to show up is to do the install step again and that always brings up the error message.

Don't know if that info helps but I'm out of ideas and ready to give up on booting onto the external card and turn this thin into a really expensive GPS!

G'night

Tom