Don't think I can help much. I have a ReadyNas Duo connected to the Apple Airport Extreme router via ethernet and also an iMac, also connected to the router via ethernet. I used to have the duo purely as a NAS and it was always present in Finder. Actually, it showed up two or three times, I think this was due to the different protocols that were in use and as I didn't understand them, I never turned any off.
However, now I use it purely as a Time Machine disc, using the time machine settings that are in the Readnas interface. I turned all the protocols off apart from the one that it says it needs.
The ReadNAS duo only now gets mounted when a backup is being done...then it dissapears. Occasionly it says it cant connect as it doesn't have the password, which is rubbish, cos telling it to go again and its fine.
Can you still view the readynas interface via the web browser? I'd go in and double check the settings, and also check your available space...I dont know what happens when the disk is full (I thought it would overwrite the oldest data).