Still no joy. Whatever I do it doesn't work. I have formatted the drive in Windows, on the command-line, I've made sure it has an MBR, I've re-partitioned it, I've tried diagnostic tools, all of which tell me there is no problem - there really is nothing left to do.
It is odd that the internal drive is completely accessible from Windows and from the Mac - I can write to it and Calibre can even read books directly off it. It would make a perfect removable drive. The only problem is that the Opus itself can't seem to read from it or write to it!
What next, I wonder. I would have thought Bookeen would have got back to me today :-(