The MacBook Pro could not see its internal HDD. We originally thought the HDD was bad, but after swapping with a drive we know that works, realized the issue was related to the HDD cable.
Thanks to the easy to follow repair instructions, this process was extremely easy.
The new cable was just slightly longer than the old one. So when removing the stickers on the tape, I recommend removing one at a time, and slowing pressing along. This way you ensure it lays flat and doesn't kink in the middle.