DVD/CD burning problems

[Grey_Wolf]

In attempting to first burn a back-up onto a DVD-RW where I already had finished files the burning program warned me of potentially owerwriting the already finished files instead of just adding the one file I wanted to back-up.

On attempting to burn a finished file onto a CD-R where I also had other finished files there was no warning but the CD was ejected from the burner.

Does that mean I can't make a back-up copy of a worked on file? Or add a finished file to a disc that already contains finished files?

How do I go about either backuping worked-upon files or saving finished files onto discs?