Make Mirroring Default

If you are referring to moving the menu bar to make the external display the main display, you have to open System Preferences Displays pane. Once you manage to do that with a shattered built-in display (hopefully you can see something on it), you will have a System Preferences Displays window on each screen. Click on the +Gather Windows+ button on the external display's window. One the window for the built-in display, click on the Arrangement tab. In the representation for the two displays, drag and drop the white bar (representing the menu bar) to the other display. The location of the menu bar should change and become visible on the external display. You can also change the relative positions of the two displays so that there is the least amount of disruption from having a display that is not useable.

There is also a checkbox for +Mirror Display+ on the Arrangement tab. I would only use that option if the native (optimum) resolution for the external display was exactly the same as the native resolution of the built-in display. If the resolutions are different, what you see on the external display will most likely be fuzzy.

Here's the keyboard sequence, from immediately after you startup.

Control-F2 <- Puts focus on menu bar with Apple on menu bar highlighted.
Return <- Drops down Apple menu.
Down Arrow key 4 times <- Highlights System Preferences on Apple menu.
Return <- Runs System Preferences.
Type the word displays and then high the Return key

At this point, you will hopefully see the Displays pane pop up on the external display.

On the System Preferences window for the built-in display, click on the Arrangement tab. In the representation of the two displays, drag and drop the white bar (representing the menu bar) to the other display. The location of the menu bar should change and become visible on the external display.

Now that you can see the menu bar and the external display is now your main display, you can use the iMac.