Bmow: I wouldnt use a relay. But I know too that its just a concept ;-)
I would use a 4053 mux or for low r-on use a max4619.

Click to expand...

But do the transistor-based solutions actually work? That's what I'm struggling with. For example, the 4053 datasheet says it has a maximum "on" resistance of 190 ohms at room temperature. What's going to happen if there's a 190 ohm resistance between the Mac's ground and the Discovery Board's ground? Will it still function as a return path? Will you end up with one ground at a slightly higher voltage than the other, skewing the thresholds for the digital signals? That's why I suggested a relay, for a true hard-wired ground-to-ground connection.

Edit: I should have read your entire sentence. So the Max4619 has an "on" resistance of 10 ohms. Is that low enough? I'm not sure how to determine it.