Ends 7/22. Staples has the Seagate Backup Plus STDR4000901 4TB USB 3.0 Portable External Hard Drive (Red, Black, Blue, or Silver) for $110 - $25 off when you pay with Visa Checkout = $85 with free shipping. Retail price: $180. Has excellent Amazon Reviews.
eBay has the WD 4TB External USB 3.0 Portable Hard Drive (Black) for $100 with free shipping. List price: $200. Features data transfer rates up to 5 Gbps with USB 3.0, automatic backup options, compatible with PC and Mac.