09-17-2012 12:21 PM - edited 09-17-2012 08:13 PM
Let me take a crack at this....
Portable drives are external hard drives. However many are bus powered instead of relying on the 120v power source. Note some laptops may not have enough amperage so a portable drive takes TWO usb ports to run the drive.
09-18-2012 07:45 AM
09-25-2012 02:58 PM
All portable drives are external drives. However, not all external drives are portable (enough).
Some external drives are the bigger, desktop-styled drives that required AC power in addition to the USB cable. While you can bring it along, you won't be able to run the drive without external power source; so forget about running them on a plane.
On the other hand, portable external drives are designed to power via USB. Therefore, the USB cable is all you'll need to run the drive with the computer.