Thanks, tompe. Unfortunately this doesn't appear to be happening. Each time I switch on I get asked for the language, I select English, then on the next reboot it asks me again, so I guess it needs a config.dat file in which to store the settings (this file does not exist). I also seem to remember font files, but these are no longer there either.
If I look into the folder when the drive is mounted on my Mac desktop I can see the ebook folder and Calibre has put books correctly into this folder, using folders for author, title etc., but the Opus doesn't see the books. If I opt to view using folders on the Opus I see an Adobe Digital Editions folder, but since ADE doesn't see the device I can't do anything with that either.
I'm pretty convinced that if I can reinstall the system folders and files it will work again.
Thanks for your help.