Thanks Macdrone and waynestewart.

Yep standard troubleshooting, PRAM nvram, logic board didn't make a difference.

Just makes sense that the "alternate" bus (the working ATA-66 in use) might be bus 1, rather than 0.

Wish I understood more about reading the ATA device tree. mac-io/ata-4@1f000/@1:0 (or @0:0)

Looks like diskpart on the PC side explicitly lists Bus #.

Guess I'm going in and just pull the drive in question, and do it externally...

Thanks again.