Seagate FreeAgent Go 880GB 2.5" USB Portable Drive for $129.99 shipped after coupon code EMCYZNT39.  Here is a screenshot for reference.  It is USB powered and does not require an external power supply.  It is smaller than most external hard drives, making it more convenient to carry around.  Note that 880GB is currently the largest capacity for a 2.5" portable USB-powered external drive.  Most places have it for about $149.