iMac not working as external display
I have a Macbook Pro and a late 2013 27" iMac, both with Mini Display ports and both running OS X Yosemite 10.10.3. I've connected both of them using a brand new Apple-brand Thunderbolt cable, as per Apple's instructions. You're supposed to be able to press Command F2 on the iMac to put it into Target Display Mode. But when I press Command F2 nothing happens. At all. Every function key works. Both computers work. All keys work. F2 works as a function key (brightness). But the Target Display Mode isn't working. Any ideas what I can try to fix this?