All USB ports have an upper limit on how much power they can provide - 500 mA current, I believe. Early external drive units, especially those with lower capacities and those made for portable use with laptops, often employing 2½" internal drive units, were able to run within that limit. As larger drives were developed, more recent ones have required more than is available on a USB port. One solution used was to provide cabling that plugs into TWO USB ports and use those combined supplies to run different parts of the external drive's electronics, so the external unit still can operate without its own power supply. On those units it is CERTAINLY recommended that you plug into BOTH USB ports to ensure the unit receives all the power it requires. The alternative solution that dominates in larger desktop external units based on 3½" drives (and also used in some portable external drives) is to provide its power separately and NOT draw any power from the USB port.