Even as it is external, try this sticky:Guide: If you installed a new disk but the system can't see it
Just reread your post. Maybe cause it a laptop, and underpowered. You may not have enough "juice" in the laptop USB to power an external drive.
I've had that happen, a laptop can't power a 3-1/2" hard drive enough, but can a 2-1/2" drive, through USB.