I'm certainly no expert, but I've never been able to run a spinning disk HDD off of a raspberry pi without a powered USB hub. I imagine the thermals and form factor would be hurt pretty badly by including a plausible power supply.
faster-spinning-plates would draw more power, I guess.
I was also trying to run it as a NAS, so it had a decent amount of IO.
Maybe it could have been fixed with a better power supply, but switching over to the powered hub was easier to figure out.