I was just about to ask about the eject mechanism.  If it ejects with a paper clip (triggering the eject mechanism, right?  with the case on my PM 7100 removed I see a button, essentially).  If the auto-eject is driven by a motor, then perhaps there is an issue with the motor.

When you boot from an external disk, what happens with the disk in the internal drive?  Does an initialization pop-up come up?  Does it not recognize it at all?  I also forget if you said - does it sound like it's trying to eject or not at all?  Because I suspect there are multiple possible problems if it is an issue with the motor.

And then with the eject issue I would probably check the motor - if that's all it is, since I think the eject mechanism itself works, just not the auto.  I don't know if logic alone can solve that...