since upgrading to yosemite my MacBook Pro want detect my external display

I was just mirroring the primary display. I can try configuring it as an extended display. It was also an LG TV rather than a monitor but that shouldn't make any difference should it? I do also have a BENQ monitor with a HDMI port which I'll also try.

The LG TV is fine for testing, I tried with my Samsung large tv and had the same problems. Specifically the problem is that if I use the large screen or tv as the primary display, I can not boot up successfully. I usually like to use my large screen as my primary display and use the lap top as a secondary screen. So that means when I turn on my computer everything is done on the large display. The lap top display is only used if I want to look at two different programs/document simultaneously, one on each screen. The way I get around the problem is to disconnect my large display, turn on the computer and log in. The reconnect my large display. Alternately I can set my large screen to secondary or parallel prior to turning off my machine. Then I can boot up and change the configuration. Neither of these is acceptable to me.

My issues are even worse than some of these. I ugraded to 10.10.1 from Mt. Lion, and everything worked fine for several days.

Then, the system would go into a loop, just the spinning cursor, never breaking out. I'd power down then reboot. I'd work a while then another loop/reboot. Then , and rebooted. More loop/reboot. Then I couldn't reboot in normal mode, but Safe mode worked. I then repaired several bad permissions. That worked for one loop/reboot cycle and then I couldn't reboot at all in Safe mode, nothing worked.

Then hooked up to my backup image (SuperDuper) of Mt. Lion, and ran Disk Utility, and all I could do was run repair disk. No help, still couldn't reboot.

Then tried reboot verbose and suddenly that worked, even though nothing had changed.

Doing more investigations and found the problem with displaypolicyd. Now that I was on the mounted Yosemite drive I repaired the permissions, rebooted successfully, and noticed that the permissions got whacked again.

Repaired the permissions, and did a cold boot, everything comes up as expected. Hopefully this is the solution until Apple fixes this guy! I haven't done any serious work on the system yet, but I'll repair the permissions after I reboot and hopefully that will also fix the dead loop issue as well.

My system is MBP mid-2012, two external monitors with the clamshell closed and I'm using DisplayLink with the new beta driver for Yosemite (2.3b).

i have this issue since 2 years, after upgrading from 10.8 to 10.9.

it still persists after clean installing 10.10.1, 2 month ago and erasing the disc before with NOT migrating any old data to the new installation.

when i start my imac 27" late 2013, my second NEC display is not detected by default, i press on/off and then it is detected, but with 10.8, the behavior

was different, the imac detected the NEC display automatically when booting.

also osx 10.10.1 will not save the desktop image of both displays (plain grey), after booting it shows up with the default yosemite "mountain / hill" image.

terminal error, wont disappear, after repairing several times :

"

User differs on "private/var/db/displaypolicyd"; should be 0; user is 244

Group differs on "private/var/db/displaypolicyd"; should be 0; group is 244

Repaired "private/var/db/displaypolicyd""

thanks

I have the same issue...no ability to utilize a second monitor since the Yosemite update. I've connected three different monitors now, all working on other setups, with no success.

I have a MacBookPro 2009 and updated to Yosemite recently.

Now i have issues with my

external monitor - Dell U2713HM

It was working with DVI cable for a week at max res of 1920x1080 ( well below its usual resolution)

But now the DVI to mini display port is not even being seen. ( suddenly stopped working )

Tried a different cable but still DVI cable to mdp is not seeing any external monitor.

(I currently have to resort to vga to my mini display port)

What gives ? Shoddy workmanship Apple ? Seems likely. If I find a fix I will report.

Same problem, MacBook Pro (13-inch, Mid 2009). To get the external monitor to work I have to unplug the mini-display plug after boot-up - then it will come on. Making it the secondary display does not help. While this situation is workable, it's not acceptable, and eventually either the plug or the socket will pack up from too much in and out. It is about time Apple diagnosed the problem and fixed it. I was on Snow Leopard and needed to upgrade to use new software and with 8GB of RAM and having used Yosemite at work I figured there would be no problems. I supposed it could be worse. APPLE, PLEASE FIX THIS!!!

i also tried all things mentioned before.

i always turn on and off the macbook then it always works, no need to disconnect. still annoying.

fix would be great apple!