Our Apple Macintosh G4 Powerbook Titanium running os x (10.4.9) won't play audio or video CDs or DVDs, although the directory does appear ok, and all data CDs / DVDs work fine. We also can't copy files (audio or video) from discs we burned ourselves on other computers - attempts to do so result in a 'read error' message. iTunes recognises audio discs, loads, downloads title info from commercially produced audio discs OK, but pressing the play button results in nothing happening (timer sticks at 0:00) ..... Program and data files burnt on our other machines copy off fine onto the Powerbook. Why might this be?