Welcome to the community, NLKagan!
I'd advise you to use a different USB port on the MacBook Pro and/or try connecting the external HDD with a different USB cable as this one might be faulty. It seems like the drive is not being supplied with enough power for the system to recognize it properly.
I'd also suggest you to try plugging it to another computer and see if it will get recognized there.
Hope it helps. Good luck!
SuperSoph_WD