Probably not enough power.All USB connectors on the same motherboard usually share their power. If you are connecting USB keyboard/mouse, as their power consumption is very limited, you don't usually have much problem. But it will be a different story if you are connecting USB HDD, which can drain the power supply. In this case, you may have to attach external power supply to your HDD. There are two ways: - Some mobile HDDs provide an additional PS/2 connector with the normal USB connector. You may connect the PS/2 too to your computer, so that part of the power requirement can be obtained there. - If not, you may have use a transformer and connect your mobile HDD to an external power supply.