Port Compatibility

• Ports are where devices are plugged into your computer. As of August 2010, a 500 gigabyte (GB) external hard drive provides connection via at least one of the following port types: universal serial bus (USB), eSATA (external Serial Advanced Technology Attachment) or FireWire. Before purchasing an external drive, check the ports on your computer or laptop. Take note of what you have available and then buy your drive accordingly.
  
  

Drive Speed

• External drives commonly come in one of two speeds as of August 2010: either 5,400 or 7,200 rotations per minute (RPM). The latter speed is faster but takes more battery power when plugged into a laptop. Slower drives are cheaper, but not by a significant amount.
  
  

Physical Size

• External hard drives come in either 2.5-inch or 3.5-inch "form factor" sizes as of August 2010. However, these refer only to the width of the hard drive that is inside the external enclosure, so the actual size of the entire drive unit is not the same. The 2.5-inch drives use laptop hard drives and are easily carried around in a laptop bag, because they are thin and light. Many 3.5-inch drives, which use regular desktop hard drives, are designed to stay put on your desk. They often come with a stand to keep them upright and prevent overheating.