I have an iMac 15" with OS 10.2.6. I tried to add a file to a CD-RW that had one 50MB file already on it. I got an error message telling me that the disc could not be modified. The error code was "30" which, according to MacErrors, means "request user to reinsert off-line volume." I was also unable to move the existing file from the CD-RW or modify the information in the file. Is this just a case of a damaged disc or is there something else I need to do or know?