Stan,
Everything depends on the connection type, and the speed of the drives, themselves. Same for a USB 3.0 to a fast external.
An eSATA connection to a fast external will be just fine. A USB 2.0 connection to a slow (say 5400 RPM) external will lag. Same for an IEEE-1394a (FireWire 400), which will be only slightly faster.
An IEEE-1394b (FireWire 800), will fall in between those two connections.
A fast internal HDD through SATA will be the ideal, but eSATA, and USB 3.0, should be close behind.
Good luck,
Hunt
