Well, that's weird that it will only "see" pressed media and not recordable media. I've only heard of it reading either DVDs or CDs and not the other, and that's usually the sign of something wrong with the laser. You might want to even contact Plex support and see if they've ever heard of it.
If you can think of anything else (particularly a recent install of any kind of CD/DVD reading/burning software), maybe we can take a look at that... :shrug:
I got nuthin' Shoebedobedoo other than trying the old diagnostic self-test. That records a disc and then somehow reads it back. I'm assuming that it might actually record the disc during the test and then fail to read it back...if so, that pretty much means it gets RMA'd if still under warranty.
I hope so, you might even get a 716 to replace it, although for CD burning that might not be a good trade. Please let us know how you get on/what you decide and good luck.