It might help to explain the process of defragmenting.  When your computer is new files are saved whole and your free space is one contiguous area.  As files are saved, deleted, saved again, and so on, the free space on your hard drive becomes divided into little chunks.  When new files are saved into those chunks they must be divided and saved as several smaller pieces and then reassembled when accessed.  Obviously when your computer has to grab several pieces of a file from different places and reassemble it, that is slower.\n\nDefragmenting rearranges the files on your hard drive so that they are saved whole and in one place.  It also makes your free space into a single continous area.\n\nData on a CD or DVD is permanent, that is to say you cannot move or delete it.  Therefore you cannot defragment them.  In fact you don't need to for the same reasons.