It's slipped my mind that I've neglected to post anything regarding PSX HDD firmware. I was able to successfully transplant the AppCode sections of the drive firmware, however the ROM transplant did not go so well. I'll have to start over since work has been so busy I've misplaced my donor drive.

For clarity: The AppCode and ROM have to "match" so to speak in order to work. However, the ROM used was overdumped so it ended up bricking the drive. A trivial fix moving forward, just need a new donor and to try again. Sadly writing the AppCode takes several hours.