I pulled out two old Dells and one had a single IDE 'address' in the BIOS so it's HDD and CD were indeed Master/Slave on that single channel, but no pins were offered - that means a ROM setting, then. On the slightly newer Dell, it had 2 IDE channels in BIOS, and I swapped DVD and CD drives interchangeably and they were acknowledged in each of the two computers.
So the DVD had to be set as Cable Select or Slave for the older Dell to acknowledge it on the same channel that its orig CD drive used.
And the older CD inserted into the newer Dell was also acknowledged properly, so it's settings weren't in conflict, but since it used a separate IDE channel, I didn't expect that.
Were all slim-line Optic Drives set as Slaves or Cable-Selects, perhaps? Well, if they were, then Acad's issue wouldn't be an issue-! So apparently not all were 'set' with Slave settings. Spartacus will be glad to know.
But none of this solves the issue, but at least Kouryu's bright light brought out this. It makes me believe this was a trick by the name-brands to ensure future replacement-part sales. Grrr - anything to make it difficult on consumers-!