Memory Cache

• Most hard drives, external or internal, have random-access memory called cache to keep frequently used chunks of data. Generally, the more cache the drive has, the more data it has ready to transfer, speeding up transfer rates.
  
  

USB 2.0

• Many external hard drives for home and office use the USB 2.0 interface, rated to transfer up to 480 million bits per second. The real-world transfer rate is closer to 45 million bits per second, limited by communications overhead and the size of the drive's memory cache.
  
  

eSATA

• You'll find the eSATA interface on many high-end external drives. Its performance realistically peaks at about 300 million bits per second and has less overhead than USB 2.0.
  
  

FireWire

• The IEEE 1394 standard, going under the trade name of FireWire, has a speed rating of 800 million bits per second. A hard drive using FireWire may have actual transfer rates of from 40 to 100 million bits per second, depending on the model.