Portable external hard drives are usually 2.5" hard drives (laptop hard drives) in a slimline enclosure, making them easy to carry. Although they are portable, they are not unbreakable and extra care is needed while transporting them, for example, don't put them in your back pocket and then sit down!
The portable external hard drive is commonly "USB Powered", which means you don't have to carry a separate power supply unit around with you. The only downside to this, is that the available current (power) is limited to the computer it is attached to.

This may not seem like a big problem, but if you have many other USB powered devices connected to the same computer (especially when connected to a laptop/notebook), then you will find the USB powered external hard drive will not perform optimally.

Some of the portable external hard drives that are available come with a separate USB power cable, which provides an extra power boost to the hard drive if required.

source:http://www.helpwithpcs.com/buy-guides/external-hard-drives.html

Portable External Hard Drive video review