Alright I'm obviously a noob at this. Turns out I had confused the purpose of the pins... and was trying to use the two furthest from the SATA connector as RX and TX. *face palm* But at least now I see data from the hdd, and the code confirms i have the BSY issue.
I have the exact same problem! I'm wondering if the HDD itself is busted as I'm seeing some strange voltage readings on the three hdd pins (gnd, rx, and tx), but given my current lack of understanding I'm not confident enough to say what I'm seeing is unexpected. Have you had any luck?