Aha, interesting, thanks! I had no idea; I'd always thought it was from "master copy" like in, say, a manuscript that is duplicated by hand. (Sure, the life of a mediaeval monk working in a scriptorium was pretty strictly regulated, but I doubt anyone saw them as slaves, exactly.)

Though that Ampex manual isn't all that convincing in this context, IMO. It's about the hardware level, all capacitors and oscillators and stuff, in "master" and "slave" amplifying circuits. That has pretty much nothing at all to do with "master tape" per se; it's more like "master" and "slave" hydraulic cylinders in the clutch or brake system of your car.

Your HN link feels like a much better argument here. (Though I admit I haven't followed any of the links in it yet; going just by the quoted bits in the comment.)