external monitor will not work properly after upgrade to El Capitan

[BOXERGIRL3]

Since upgrading to El Capitan, my external Thunderbolt monitor wil not wake properly. I have it connected to my MacBook Pro Mid 2012. Sometimes it wakes but most not. Can you help? I have researched and don't find anything.

[dfulmer]

Was just looking for information on this type of issue. I'm working on rolling out 10.11 to ~450-600 Mac's over the summer for our k12 school district. Some staff members, <10, are using El Cap.

This morning I discovered an issue, brought up by one staff member using El Cap, that when they put their MacBook Air into clamshell mode to use their external monitor, the external monitor goes to sleep and will not wake again unless you open the MacBook Air.

I tested the issue with the same hardware** in my office and lo and behold, it occurs again. I also created a Test user account locally on the device so I could test in a known good state and the issue persists.

**MacBook Air > Apple 27" Cinema Display w/ mini-display port connection into the MacBook Air's Thunderbolt port.

[apizz]

Are there any firmware updates for the Apple Cinema Display?

[JefferyAnderson]

Do you have power connected to the computer? It will not send signal to an external monitor if the clamshell is closed without a MagSafe connector powering the computer.