May be that you need an external ps for it?
A Few drives are able to function on the power supplied by a single usb port,
Others require a "Y" cable and thus use two ports in order to get adequate power.
Still others require their own "wall-wart" power supply.
If it is not a power isssue, then I would consider trying a different usb cable,
and see if you still have trouble.
THEN, I would consider migrating the drive, either to another enclosure, or to "internal" setting.