Yosemite does not recognize external displays...

I've been struggling with this issue on my Macbook Pro Retina and my HP ZR2740w monitor for awhile (ever since an update prob a year ago), tried everything in this post and took it in to make sure it wasn't the graphics recall issue. Anyways, in debugging a different but similar issue on my monitor at work, I ran across this, which appears to fix my problem...

http://sepczuk.com/techblog/2013/07/displayport-sleep-resume-problem-resolved/

Essentially I just put a tiny strip of electrical tape to block pin 20 on the display port connector into the monitor. It's small but not that hard since it's on the end. I have one of the few certified DP cables from Accell, its a couple years old but don't think the spec changed since it was certified. I found a lot of varied info about firmware issues on this HP monitor (I got a pair of refurbs from newegg for cheap). I'm also fairly certain I wasn't having these issues until a particular update. My guess is Apple updated something, prob in spec, that conflicted with other issues. I'm guessing since they can prob blame somebody else for compliance won't bend to support other hardware, especially since I'm sure they all use apple monitors and aren't annoyed by the daily frustration.

I can confirm that I fixed this issue for my ASUS monitor / macbook air yosemite as well...although it's a slight PIA.

1. Unplug the monitor

2. Unplug the dvi connector from your macbook (keep it connected to the monitor in the hdmi port)

3. Plug your dvi connector into your macbook (wake it up if it's asleep). You'll notice the black header / footers appear

4. Plug the monitor back in

Pros:

1. It works every time

Cons:

1. You have to do this every time.

Hi,

Had the same problem after installing yosemite with a macbook pro late 2013, 15 inch

I used this firmware update for my mini dv to hdmi apple adapter:

Mini DisplayPort to VGA Firmware Update v1.0

Once you have updated the firmware you just need to restart et voilà!

I found a simple solution; but none-the-less this problem is very annoying.

• Hardware: MacBook Pro (Retina, 15-inch Mid 2015)
• OS: OS X Yosemite 10.10.5
• Problem: A pair (two) external Samsung displays are connected but not detected, even when using "System Preferences -> Displays -> Detect Displays"
• Connection: Plugged into the Thunderbolt/Mini-DisplayPort interfaces
• Cables or Adapter: Cable has Thunderbolt/Mini-DisplayPort on one end and DVI on the other.

Solution, in 14 very easy, no big-brain required steps:

1. Of course, check and adjust the display's configuration menu, and use it to make sure that the display is set to operate on the correct interface; in my case, the DVI interface, but for others it might be HDMI, etc. Also, assure that the cable connecting display to computer is securely plugged in at both ends.
2. With clam-shell open, (i.e. with the laptop open and the built in display active,) bring up the "System Preferences -> Displays" panel, so it is available for a later step. Make sure you know how to press <options> to get the "Detect Displays" button to show up.
3. Unplug power cord for both displays. This means at the wall-socket or power-bar, actually pull the power plug out of the socket. The purpose of removing power is to blank the monitor's firmware memory, in preparation for a forced reboot of the display's internal firmware.
4. Wait 20 seconds. Firmware has capacitance, and 20 seconds assures that its memory is indeed blanked.
5. For one of the displays, plug the power back in. Immediately get back to the computer controls, and to a position where you can watch what is going on with the status lights on the display.
6. There is a short time window here, where-in the display has just rebooted, and it is holding the signal (in my case the DVI signal) active while it anticipates a connection. Watch the status light on the display; when (by your best guess) the light indicates the reboot has completed and connection is live, move to the next step. However, if you wait too long, the display will time-out, which is to say: it will stop looking for a connection, and it will shift the status of the connection to a state that does not serve the desired purpose. (In that case, unplug and repeat.)
7. Now, during this short window while the display's interface is "hot", use "Detect Displays" (mentioned above).
8. This time, it should detect the display. The result is that both built in monitor and external monitor are active.
9. Shut the clam-shell, so now only external monitor is active.
10. If you have not been using external keyboard and mouse, you need to get them set up and active before performing the next step. Note that you must have an external power supply connected, because that is how a sleeping MacBook knows it should respond to an external keyboard pressing <enter>. With external power, keyboard, and mouse set-up and confirmed to be working, proceed.
11. Put laptop to sleep, using Apple menu (upper left corner).
12. For the second display, plug the power cord into a power outlet.
13. While second display's connection is active, press <enter> on the keyboard to wake up the computer. (Again, there is a time-window here; post re-boot but pre time-out.)
14. The second display should show up without further prompting. (If not, perhaps try "Detect Displays" quickly, while display interface is still "hot".)

This worked for me.

AL X