Actually today at work I tried to connect to my PC and no luck. First few times nothing, and then after trying all USB ports (I've got 2 hubs and 8 ports on the PC itself) all I got was "device not recognized". All tips on the wiki were tried. I'm a bit sad about this. I mean I can live wonderfully with just the SD card, but the 512MB of internal memory... well, why not? Plus, I don't always have a card reader with me.
ser1us: Now to your question. Try the following:
1. don't eject any of the cybook drives
2. start disk utility
3. click on the highest level device in the side bar and click the icon "dismount" or "unmount" or whatever, not "eject". That should completely remove the whole device.
If the problem rests with the cybook itself, this procedure will bear no fruit. It might be that the cybook itself decides to send the signal to reconnect. Then you're out of luck.
Good luck.