That is curious because in my experience, Mac OS is very good at keeping your standard screen as the main screen, menu bar and all, with any secondary displays that are plugged in -- via USB, HDMI, Thunderbolt or DVI -- being additional screen real estate, but never becoming the new main screen.

If you're using Keynote for your presentation, you can actually specify which of multiple screens you want as the presentation screen and which you want as the speaker notes screen (if you're not using mirroring) which can be really helpful as the speaker's screen has a clock, timer, preview of the next slide, etc. That, of course, won't affect how other apps interact with the multiple screens, so that's not a global solution.

The fix turns out to be easy, but you have to know how to do it, because by default, Mac users might know to go to the Displays system preference, but only to turn mirroring on or off.

But there's more you can do...

To fix your dual monitor problem in Mac OS, go to the "Displays" option in "System Preferences" (off the Apple menu):

Once you launch it, you'll most likely see information about each of the displays shown screen-by-screen. On my main screen, here's what I saw:

Click on "Arrangement", the middle of the three tabs along the top. Now you'll see that it recognizes two displays and how they're "connected":

To change which is the main screen, simply click and drag the tiny white menu indicator bar from one screen to the other:

That's easy enough, isn't it? How we're supposed to know it can do that, however, is a mystery to me because there's certainly no visiaul indicator here.

While we're at this point, try dragging the displays themselves around on the screen too. You can set it up however you want, actually, with the external monitor to the "left" or "right" of the main screen, or even "above" or "below" it.

Finally if you just want to have both screens show exactly the same content, choose "mirroring", though be warned that while you're mirroring, it's the lowest resolution screen that defines the resolution of the larger screen too, which can be a bit disconcerting.

Hope that helps out and good luck with that presentation!