So to recap, here are the most likely problems:
(1) PCB1 is bad. PCB2, while the same revision, is not sufficiently similar to work with HD1.
(2) PCB1 is bad. HD1 is also bad.
While I can't definitely rule out either option, I have no evidence to suggest that HD1 is bad. The drive did appear to spin up with PCB2 vice PCB1, and there are no sounds or other evidence which might lead me to give more credence to a physical damage to the disk scenario.
In addition, I found this:
Seagate tends to use many different programs for the same hard drive model. This is why it is very difficult to find a circuit board with the same program (firmware) as your original circuit board. It is much easier to find a physically identical board. A firmware transfer can then be done to make an identical copy.Also:
...it seems that newer Seagate boards have MANY different firmwares (even when they have the same firmware on the label), and is therefore very hard to match.The argument here suggests that perhaps PCB2, while listed on the drive as the same firmware, may not actually be the case. So using the service to "clone" PCB1 would ensure that the replacement PCB does in fact have the same firmware (instead of just assuming it does in the case of PCB2, despite the label).
The other option is to take a closer look at PCB1 for signs of damage that may be able to be fixed. This is certainly something that I can't do myself, but some friends have offered to help and I am grateful for that.
So the immediate plan of action is to do just that, examine PCB1 for signs of damage in the hopes that maybe a broken solder point can be fixed. If that doesn't work, the cloning service (at the cost of $50) seems like a good next step.
What are your thoughts, especially regarding the cloning option?