This primarily depends on your setup and, as the others noted above, what may be running in the background and the configuration of your IDE devices. If possible, leave your hard drive on its own channel if you are burning straight from the hard drive. Also, CD-CD copies, depending on your setup, may have to run at a lower speed. The buffer underrun error you are receiving means that one of your components can't keep up with the others. By changing your IDE configuration or by closing any programs that don't need to be open, you can free up system resources and you may have a better chance of not getting errors at your top burning speed.