I have a G4 with OS X 10.4
I was burning dvds and the driver stopped reading the blank disk, it reads all other disks just not blank disks.
If I insert a blank disk it will say tray doesn't have a disk, insert a media recordable disk. and spits the disk out
Try launching Disk Utility (in Applications-->Utilities) and run Disk First Aid to verify if there are any errors. If so, you might have to boot from the Mac OS X install CD to run the repair option. Just insert the first Install CD, restart the Mac, hold down the C key after you here the chime, and once you are greeted with the install window, go to File and select Disk Utility (or "Repair Disk if it says that...my memory fails me at the moment). Once Disk Utility comes up, run the Disk First Aid option to repair the hard drive.