Honestly, I'm thinking it's not a CHS drive at all. I read it in PIO mode, and it seemed pretty normal (though there's no utility for it). I just finished imaging the patient one and no bad sectors so I'm honestly not sure. Perhaps it's just a software corruption case complicated by the PCB not being original. They are concerned about it needing to boot, but it's hard to know for sure if it will.
Could be that the drive is just a bit flaky and the PC-3000 kept it stable enough to read, or could be logical.