The Computer with no name has passed...

[Ghoulish Delight]

SATA controllers will address the device at the speed it supports, so it would communicate with the DVD drive at the 1.5gb/s rate and the hard drives at the full 3.0gb/s rate.

According to Wikipedia:

I did some tests with my own system, using SiSoft Sandra and Everest, and even when burning a CD, the HD (a Sata/300 160gb 7200rpm drive) performance didn't drop off during benchmarking.

Were the burner and the hard drive on the same SATA cable? The info above does not contradict the info Moonliner posted. A SATA cable/controller may be able to handle a single device that's rated at a higher speed, but that's different than speed negotiating between multiple devices on the same link. It may not suck all the bandwidth, however it may only be able to handle a single clock rate.