[krokodyle]

The pen dye can seep through the disc surface and damage the data. Maybe not today, or tomorrow, but after a few years. And even if you use pens that are "CDR safe", it's still not the best idea if you value your data.

You *can* write on the inner ring, or the outer envelope/case, or print a directory of the disc & use that.

Oh, and attaching those round labels to CDRs and DVDRs can also be risky, though I know that many people do it.