Even though many of today's notebooks come with spacious hard drives, you can always use more storage. Large multimedia files can chew up disk real estate in the blink of an eye. Or, maybe you want the ability to back up your hard drive--or at least part of it--on the spot. Luckily, the latest high-capacity portable hard drives are extremely light and small enough to fit in your pocket. They're also easy to install and use with any Windows or Mac system. We tested six 5,400-rpm drives in order to select the best one for taking your data with you wherever you go.