Yeah, "primary"/"secondary" is common. For hard drives "master"/"slave" isn't even an accurate metaphor, since both are driven by the host computer, it's only that the secondary is physically wired to the primary and can't use the communications channel when the primary is in operation. The primary doesn't send commands to the secondary or anything.

See also this comment I wrote a couple days ago re ZFS avoiding the use of "slave" (which was used in a different sense from "slave" hard drives, and both of which are different senses from the "master" boot record): https://news.ycombinator.com/item?id=23521392

In general I think it's good that people are picking more meaningful terms.