If you regularly work with a multiple-display setup, you might have to reassign a primary monitor if you change equipment or a monitor dies. It's easy to do, but not intuitive. Vincent Danen tells you how in this tip.
If you've ever had the pleasure of using a computer with more than one display, you know the advantages of the extra screen real estate. I've had the pleasure of using a multi-monitor setup for the last ten years and it has spoiled me to the point that attempting to work on a single monitor is often an exercise in frustration. For those who don't work with two or more monitors on one computer, that may seem entirely odd and perhaps a little snobbish, but ask anyone else who also uses a multi-display system and they will make the same claim.
Multi-display setups on the Mac are easy. Most Mac laptops will allow you to use the laptop display and an external display so getting a dual-head laptop setup going is dead simple. With my Mac Pro, I outfitted it with a second video card so it's capable of running four monitors at once. This gives me great screen real estate and makes working in front of this system for 10+ hours per day really enjoyable.
The downside of so many displays and the ease with which the Mac handles them is that if one monitor dies or is disconnected, particularly if it is the primary monitor, OS X reconfigures everything and the menu bar may not end up where you want it (the menu bar traditionally is only displayed on the primary display, as is the dock). For instance, the primary display of my system was the lower left monitor, and it happened to be the one that died on me recently. OS X's reconfiguration left me with a dock in the lower right monitor, and the menu bar on the upper right monitor. When the new monitor was plugged in, OS X did not reconfigure it; it didn't have to -- the primary display was set and available.
It's really simple to re-assign which monitor is the primary monitor, but is not at all obvious. To do so, launch the System Preferences. Here, click on the Display preference pane.
In the Arrangement tab, you can click on one of the monitor representations and drag it around in order to set the physical location of each monitor. You can also click and drag the white bar that represents the menu bar. In my case, I had to drag the white bar from the upper right monitor representation to the lower left. Once this was done, OS X refreshed the display and the menu bar was in the right place once again.
For other options with the menu bar, you can use a shareware tool like Menu Everywhere, which will let you place the menu bar on any display and can even embed it into application windows. Another useful application may be DejaMenu, which is a free application that lets you open the menu bar as a contextual menu on any monitor.