There is usually a very distrinct trade-off when it comes to choosing an external hard drive. You can go with portable, USB powered but low data capacity or choose a bulky, high capacity option that needs mains power.
It is for this reason that WD’s new 2TB My Passport excites us so much. It packs a USB 3 connection which powers the drive.
Plus it manages to deliver great transfer speeds, with small file copies staying over 90MB/sec throughout the entire transfer. It isn’t cheap, but it is an excellent little drive.