There is a flex cable mounted to the arm with an IC mounted on thr flex cable and wires connecting to the heads. So I assume this IC is a preamp or something. There is no other circuit, no switch.
In the photo you have accurately depicted the stop. There is not a recess on the arm.. a flat surface on the arm bangs up against that stopper.
The IC on the flex cable is indeed a head preamp.
You are correct, there is no recess on the arm. The round protrusion is to ease adjustment of the physical stop - It's main purpose is to prevent the heads from errouneously being driven off the surface of the platters, which would be devastating. Without this stop it would be possible for a malfunctioning controller to effectively decapitate the drive.
Then, given I have swapped the entire controller board with a working drive, and the track zero issue follows the hardware not the controller....
This brings up a great point. If the problem was entirely restricted to the controller, then swapping controllers would have fixed the issue. Something mechanical must have gone wrong.
There was absolutely nothing in there that in any way looked like a track 0 sensor.
The only thing it could be, is if the magnetic characteristic of the armature in there is somehow not symmetric, but I kinda doubt that.
Reassembled, the stepper still spins happily and acts just like before.
So, it is still a bit of a mystery at least to me how this drive detects track 0.
I haven't been able to find any reference to a stepper with built in track 0 detection, either. Probably just my search skills.
I don't think there's any use to the armature being that way as this stepper can make one full rotation for a sweep of the disk - The detection would have to determine between a regular seek and being at track 0.
I do wonder how it detects it's stop, then. It must do it in some way.
The main example of end stop detection I was aware of was the afforementioned CMI 3212 (Mistyped model number, my bad) which includes it as a cam inside the stepper. I've not seen any drives with physical limit switches, external sensors are all optical (Several Seagate drives, all Miniscribe stepper drives, most Tandon drives, etc).
Note that the stop has a little "tab" projecting.
In case you haven't had the displeasure of hearing it before, drives slamming the actuator against their end stop makes a very audible "clack" or banging noise. These drives would be an awful lot noisier if this was their detection method. If you would like, I can even video my unit doing seek tests where you definitely cannot hear it hitting the end stop. I might even be convinced to lift the lid on it again and stick some shims in to show that it stops just early of the mechanical stop.