Minutes later, when the burning is complete, an error message in the status window: "All data was not saved".
I said "What? Nothing was written?!?"
And then I remembered: Picture the programmer as a Freshman English student. You know, the kind who writes things like "from whence", and "however it happened" when he means "however, it happened". And then it all makes sense: the programmer meant "Not all data was saved." I.e., some of the files (which it told me about) were skipped.