Trying to burn a CD on my iBook; but when I insert a blank disc, the combo drive hums, makes a terrible sounding grinding noise a few times, hums a bit more then ejects the disc. The Finder won't even recognise that a disc has been inserted. I've tried at least 3 makes of CD-R; and it can't be a software problem since the Finder blank CD dialog doesn't come up asking me what to do. The curious thing is that CD-ROMs, DVDs, audio CDs, and CDs that I previously burned myself all work fine in exactly the same drive. Any suggestions?