Simon, thank you for taking the time to help. Yes, that is the error I see after inserting an audio cd into either Plextor drive resulting in both drives continually hunting the disc. I cannot play a music disc with any software player:
Rhythmbox -- no disc seen
Aqualung -- no disc seen then hangs until I remove the disc (does this with either Plextor)
Gnome Mplayer -- no disc seen
Grip -- no disc seen
Again, as I mentioned in my original post, inserting a data disc into either Plextor drive yields no issue!