I think it may be a problem with your burner. The CloneCD guide says in terms of your "burnproof" functionality:

[CRX811] Drive offers a buffer under run technology like BurnProof or JustLink, but this option does not work with CloneCD RAW writing and MUST be turned off!

So to answer your question, ensure that nothing is running in the background while you burn that will keep the buffer lower than 98%. This will also require a burning speed of 8x or less and you should, of course, ensure that your burner and HDD are not on the same IDE channel.