I'm not a mac guy, but I would think an uber expensive system like a mac pro would come with at least one eSATA port.
Or does Apple not support eSATA yet?
Just looked the mac pro's up... Nope, no eSATA from the factory... what a crock.. Both my old computer with a Gigabyte board and my new computer with an ASRock board have eSATA ports built in... and I didn't even specifically look for that feature when shopping for a board.
In that case, get an eSATA card and an external drive that has an eSATA port on it. It will be just as fast as hooking it up to one of the SATA ports inside the computer.