I've tried every combination of putting cd's in the burner and not, as well as removing all other devices from the firewire chain, and I did actually get it to work, here's the whole story...
Start a backup going. Stop the backup. Try to start another backup, no joy. Under Devices, the cd-r shows up, but after you go through the incredibly slow process of starting an immediate backup, at the point where retrospect asks you to insert a disk, it fials to see any media regardless of weather there's a blank cd in the drive or not.
If I delete the preferences, then it works fine, up until the point where I stop a backup in the middle. I haven't tried to see if I can make two backups in a row.
I think it's a bug.
I also think retrospect's support of this particular drive is a bit less then perfect. Apple's DIsc Burner, and Toast have no problems at all.