Although, the 2.5' hard drives may not have the same performance as the 3.5' desktop drives, they are an excellent choice for data storage on the go. Currently, you can find 500GB of 2.5' SATA hard drive as cheap as $110. It's hard to argue any other better means of portable storage. Simply put a 500GB 2.5' HDD to an enclosure and it's ready to carry any data you wish to throw at it.
There are many manufactures that sell 2.5' SATA enclosures. Often, these enclosures are basically the same. They are a tad bigger than the hard drive and usually are made with aluminum for better cooling. They usually will come with a mini or full USB port for data transfer. Some manufactures even have an enclosure that will also be able to use as internal drive with a simple dock.