Dual External H.D.D, one is appearing the other isn't.?

I own two Seagate External Hard Drives, one is a 500Gb Free agent go (single usb 2.0 cable serves as both data and power). The other is a brand new 2Tb Free Agent Go flex Desk (powered by a separate power cord through an outlet, and a usb 2.0/3.0 cable for data). When I plug one in it appears but when I plug the other in it does not. It doesn't matter which order I plug them in the second one never appears, my system beeps to let me know I have inserted something into a usb port when I insert both plugs so i know it isn't that the devices aren't connected. I have a windows 7 laptop with 4 usb ports, one of which has a 4 port usb hub plugged into it. If you need to know anything more to help just let me know.