Read throughput is typically limited by the interface. eSATA is the fastest option, as you can see with various products that do very well in this chart. However, USB 2.0 maxes out at approximately 33 MB/s in everyday use, meaning that we would expect a maximum that is close to this value. FireWire is not very popular anymore in the PC world, but still important in the Mac community. All of the portable drives we tested are based on USB 2.0, and you can see that Fujitsu and Samsung are the fastest, followed by Western Digital and Seagate. Keep an eye on average and minimum results if you intend to make intensive use of your new portable drive.
Write performance typically is a bit slower than read throughput.