Amazon has a whole bunch of Seagate hard drives on sale for Prime Day, including internal and external drives. A particularly good deal is the Seagate Backup Plus Slim 2TB on sale for $60. This is the all-time lowest price on this hard drive. Since early June it has sold around $77 and before that was selling around $70. Either way, the $60 price tag is worth a look.
It's powered solely through the USB and uses USB 3.0 for the fastest speeds. Several models and colors are on sale, including the much larger and less portable Seagate Backup Plus 8TB desktop hard drive. I'm partial to the Xbox Green one myself.