When I connect VGA adapter my second monitor becomes primary and MacAir monitor turns off It's been over a year since I successfully connected my external Samsung SyncMaster monitor to my MacAir as a mirror or dual display. Everything worked fine.

Yesterday I took my MacAir and the VGA adapter to a meeting room and connected to a projector which mirrored my MacAir desktop.

When I got back from the meeting and connected my SyncMaster monitor, my MacAir display goes black and SyncMaster becomes primary.

The interesting thing is if I first connect the VGA adapter to the Mac, then connect monitor cable to the adapter, the SyncMaster doesn't go live. Simply displays "No signal".

But if I first connect monitor cable to VGA adapter, then connect adapter to the Mac, the Mac display goes black and monitor becomes primary.

How can I fix this?
I went back to the same meeting room, connected to the projector, and everything works fine. Somehow it has changed the default screen to the projector. Changed it back, and now I can successfully connect my SyncMaster monitor.