Q: 10.9.3 kills third monitor

I think it installs the latest version of base operating system. It it came with Mavericks you will get 10.9.3.

It it came with Lion it would go the latest version of Lion.

10.9.4 beta 1 solved this problem for me!!!! i have now my third monitor back with 10.9.4 beta1 !!!!

Worked for me too.

And you don't have to be a developer to get it. See http://www.redmondpie.com/you-can-now-download-os-x-betas-legally-without-a-deve loper-account-heres-how/

maybe as they open betas to the public and have longer and more testing and testers to expose issues and questions - Windows 7 had 10s of millions of users use public beta, 8/8.1 had over a year of public before released.

Better that then have 6 months and three releases that are more like final testing stage.

The window to get the 10.9.4 beta seems to have passed. I signed up and the only thing offered is to start with the Yosimite beta next monday.

Anyone got a direct link to the 10.9.4 beta?

SOLUTION - If your only needing two monitors , buy a Mini display jack and plug that in. The System (10.9.3) will sense the new monitor plug and allow two of three thereby allowing two monitors working

This works for those of us that only want to run two monitors in 10.9.3

Please look at the thread, or at least the title of the thread.  All of us know how to run two monitors, 10.9.3 does not recognize the third.  Thanks for trying to help, but you risk getting flamed at...

I realize that you started this thread based on losing more monitors then 2. But, there are others that had 2 and after the update lost one of 2 and were left with one. I'm offering a solution to those people.

IF you think I should have some speceal thread just for those that lost 1 of 2 original monitors that's fine but the same update effected others with similar, though maybe not exactly, the same as the original post.

If you are using modern monitors, check them carefully, looking for a Displayport jack. The black cylinder Mac pro will support up to 6 if you connect then with a simple Mini DisplayPort to DisplayPort cable.

if you actually read through the 5 pages of postings above, 10.9.3 has broken support for multiple monitors. It doesn't seem to matter if it's DP or not. It's just broken.

I read previously and have now re-read all the items posted on this thread. In every case except ONE, the third and subsequent display, previously supported with an adapter to a "legacy" display format is what was broken by 10.9.3.

And lllaass reports that contrary to that ONE report of a second display conking out, their own setup is working fine with three, when two are native ThunderBolt or DisplayPort.

From this I infer (and so stated) that if you can get back to TWO on "legacy" adapters, that may perk up and start to work again under 10.9.3.

What seems to have broken, and badly, was the support for more than two "legacy" adapters that previously worked when some were ACTIVE adapters.

Trying to get 10.9.4 to fix the problem. As it seems to have worked for us on a 2013 MacPro.

The AppleSeed program does not show me the 10.9.4 update.

Any thoughts?

I think they pulled the 10.9.4 program to start a 10.10.0 program.

I finally found a copy of 10.9.4 beta and can confirm that it has fixed my third monitor issue.