As fihart says, you need to use a hub that plugs into the wall socket.
USB devices such as portable hard drives can draw power from the USB port so that you don't need to plug them into their own power outlet. But the USB port can only supply a certain amount of power - if you try to draw too much you get the message you described.
Unpowered hubs are a particular problem because you can plug several devices into them and they all end up trying to suck power out of one port on your computer. That works OK for very low-power devices like keyboards or mice, but for power-hungry devices like hard drives it won't work. A powered hub solves this by supplying it's own power to the devices without having to take it from the computer's port.