... do you have to plug in an expansion card to use an external SATA drive?
There is an unused SATA connector in the lower optical drive bay, I assume there is some sort of adapter so you could use it with an eSATA bracket.
That said, why bother - you can get fast eSATA cards for little money, like the Maxpower models at OWC
Look for one that is hot-swap ready , maybe supports port-multiplying.
External drives are usually connected by eSATA , internally there are mostly SATA ports - same technology, just different cables basically .