First post in quite a while. Anyhoo. I'm currently downloading the Yosemite beta onto our MacBook Air that doesn't get that much use. I want to use my 27" iMac as a 2nd display for the Air so that I can experience Yosemite on my iMac display (Don't wanna run my main home computer, the iMac, on Yosemite just in case). With the 2 devices connected through Thunderbolt I can enter the iMac into 2nd screen mode using the CMD + F2 keys. The iMac works fine and dandy as the 2nd display however I cannot get the thing out of 2nd screen mode by pressing the CMD + F2 keys like it should. The only way I can regain control of the iMac is by turning it off at the power button Am I doing something stupidly wrong? Any advice?