Solvent-based inks, such as used in the Sharpie, can penetrate the protective layer on the label surface of CD discs. The formulation of this layer varies; some being more resistant to solvents than others. I am glad that you have been lucky, but do not recommend use of such pens on discs that are not easily replaceable.
Jerry Media Sciences, Inc.
Mike -- mrichter@xxxxxxx http://www.mrichter.com/