I just bought a new iMac 21.5 system. I had an older iMac and I want to continue to use, but as a secondary monitor. I connect both with a thunderbolt cable. I can not get the New iMac to recognize the older one. I opened Display Preferences and there is no "Arrangement" tab. I tried the command+F2 still no result. I restarted both computers and again no result. Any suggestion's