the attempt to burn a disc failed. The burn failed because of a medium write error

Could be a bad disk or maybe the lens needs a cleaning. Use a DVD cleaning disk. Some folks have had success with carefully inserting a credit card wrapped in a microfiber cloth.

I have the same problem, on my iMac 27", on my MacBook pro unibody. They don't burn any DVD-rw. I don't think it is a problem of bad discs, but a problem of firmware. Maybe Apple set its firmware to prepare people to not use cds anymore?

Did you find fix to your error. I am getting the same error message