Apple Mac Pros have firewire ports on both the front and back of the machine. While convenient the firewire port in the front of the tower is not well suited for high-performance tasks. Whereas the back port has a direct connection to motherboard the front port is connected to the motherboard through poorly shielded cable. This is not a problem for most uses of firewire, but high performance digital backs use the absolute maximum capabilities of firewire devices. For instance a Phase One P65+ is able to push a 60.5 megapixel true-16-bit raw file across the firewire channel at one frame per second. With that level of performance any weakness in the firewire channel can cause issues. That is why we recommend only using the rear port of a Mac Pro tower.
For more suggestions on tethered shooting see our Ideal Tethered Setup article.