Are there external hard drives that are powered from the usb and dont need plugged into the wall??
Yes, but only with 2.5" (laptop) and smaller drives. Which means less capacity, more $$$, and usually slower speeds. But if you require that level of "shirt pocket" portability then that's all the choice you've got.
Standard 3.5" HDD's draw too much power to be powered by the USB port alone.