How to run a program on an external hard drive Macbook pro Retina, 13-inch, Early 2015

I have a Seagate xbox 4 terabyte external hard drive. I reformatted it. No idea if I did it right or not. I'm trying to download the bitcoin Blockchain onto my external hard drive. I download it to the hard drive. When I open bitcoin core everything opens and then it starts downloading the Blockchain. When downloading it takes storage from my laptop and not the hard drive? How can I make this work?