Primary and Secondary makes them sound like equals -- something the master/slave relationship does NOT imply. Master/Slave is much like server/client. It implies far more meaning.
When I think P/S I think things like having multiple PATA connectors on a motherboard... while there are multiple units, they are equal in functionality and task. It's a whole different meaning from master/slave where one is giving orders and the other follows them. That said I just made a bad example as two drives on that cable are called master and slave, and they do NOT have that relationship -- unless you understand that data is often relayed by the master in that configuration, which is why you can't jumper to slave without a master on the cable.
Even just mentioning "it might be offensive" is utter and complete namby-pamby limp-wristed tofu eating prius driving special in the same way some Olympics are special bull. EXACTLY the type of thing that utterly and completely pisses me off about today's "politically correct" asshattery where you can't even say that your finances are in the black, , refer to electrical plugs by gender type, or even mention the idea of putting Hookers on a Wankel without some dipshit thinking you're being foul mouthed, insulting, racist, or sexist. Trust me, I'm going to be insulting, I'll be insulting -- like by calling people "namby-pamby limp-wristed tofu eating prius driving special in the same way some Olympics are special"
Hence why I no longer use niggard hand in selecting my vocabulary.