Could be a power issue. Is the external disk powered via USB or has it a separate power connection? What model is it? If it is powered only via USB port, is it connected to an USB 3.x port? Do you know the Amps your USB port can provide?
Could be a power issue. Is the external disk powered via USB or has it a separate power connection? What model is it? If it is powered only via USB port, is it connected to an USB 3.x port? Do you know the Amps your USB port can provide?