Question is the USB enclusure powered by an ac adaprote or does it get it's power from the USB port?
USB2 can only provide a MAX of 0.5 amps (that means device can not exceed 2.5 Watts). Some times they use a usb cable that will plug into to usb2 ports to provide additional power.
USB3 increases the amount of power available to devices.
Myself, for external HDDs I either use a powered eneclosure or I plug the HDD in to a powered usb Hub.