First, check if the issue is related to incompatible media. If, for example, you've just started using a media and the burns aren't working, this could be the culprit. "ERROR MEDIO" suggests this might be the case.
Next, try burning at 1x or 2x speeds. You'll find considerably better accuracy and reduction of errors if you keep your burn speeds low. If you're still having problems, consider using another brand for a while to test things out.
If, however, you get similar errors, you might consider visiting your local, friendly Apple Store/Genius Bar. They will be able to determine if there's a hardware error; they have the equipment and know-how to trouble-shoot the problem.
I'm interested in knowing if this worked out for you. Post in this thread, and you'll hopefully find a solution.