How portable is it? That's the paramount concern for a user on the go. Regarding external and removable storage gadgets, the smaller and slimmer they are, the better. Fingernail-sized USB flash drives and memory cards are about as tiny as storage devices get. Very few USB flash drives aren't comfortable to carry in your pocket. Some are built into stout aluminum cylinders for impact protection, while others come in keypad-protected security enclosures.
Most, but not all, portable hard drives can fit in a shirt pocket. Some drives are bulkier, especially those made with protective enclosures, but they still fit nicely into an accessory pouch in your laptop bag.
The majority of portable hard drives are bus-powered, meaning that they get all the power they need through one or two USB ports on your notebook. This is a major portability feature because it means that you don't have to carry around an AC adapter everywhere you go.
Finally, don't confuse "external" with "portable" in regard to hard drives. Although external models based on larger 3.5-inch drives have more storage space than portable drives, and cost less per gigabyte, they're too fragile to lug around. Leave them at home or at the office.