This is very frustrating. We need a fix! I have a MacBook in clamshell mode + 2 external displays and half the time when waking from sleep one of my external displays isn't detected. Here's my workaround: Put mac to sleep Turn off both displays Turn on both displays (one takes longer than the other so I time it so that both finish booting up at roughly the same time) At the right moment, when both monitors have just finished booting up, wake Mac from sleep Works ~80% of the time.