Thanks for you help guys.

The USB stick is a 16g stick as I bought it especially for this task.

I've tried re-formatting it on both my MacMini under Morphos and also my laptop under Windows 10 and I'm now fairly certain that the Open Firmware on the Powerbook still can't see it.

When I type "dev usb0: /ls" or "dev usb1: /ls" I get "ok" for both usb sockets but no list to say the disk has been found.

Not sure if I have another USB stick hanging around but I try and have dig one out and give that a go next.