Amazon currently has a good deal on a bus-powered (i.e., it runs off the computer’s USB power) USB 3.0 external hard drive at 1TB capacity for $79.99.
Details here at Amazon. (Make sure you select the 1TB from the drop-down menu circled in the screenshot above.)
As I’ve pointed out before, USB 3.0 is backwards compatible with USB 2.0 ports. I regularly use a USB 3.0 drive on both my USB 2.0 MacBook and my USB 3.0 desktop and it works like a charm.