One mini displayport on Radeon HD5870 not working after an OS reinstall; why? (Mac Pro) I've searched in every way I can think, and read forums until my eyes bug out, and have yet to run into my situation, so here goes. I have a very nice Mac Pro that I purchased pre-owned (8 cores, 32GB of RAM, super clean, inside and out). It was loaded with OS 10.8.2, placed on a SSD system/boot drive. In order to "clean house," I did a fresh install with the disks that came with, which ran 10.5.6.

The reinstall went great, and cleared some space, but I found that my second of two mini-displayport monitors (Dell P2210s) was not working. I upgraded to Snow Leopard, and it STILL isn't showing up. I know that the monitors and cables are good, because they work when I plug them into the right-most mini-displayport output on the ATI Radeon HD 5870 video card installed in the unit.

Needless to say, both monitors worked fine when I received the unit. I can see the "Detect Display" button on my "Preferences"/"Display" menu, but that does no good. I've reset my SRAM, tried hot-swapping and cold-swapping the monitor in the left-most output, and have had no results. My next step is to try using a DVI-D cable and see if it recognizes, but has anyone out there ever heard of a displayport output just going kablooey for no other reason than having done a fresh install of the OS?

The bottom line is that the right mini-displayport output on the video card works great, and the left one does not, anymore.

I tried unplugging the power cables from the video card and re-plugging them, but, just to clarify, everything worked just fine, out of the box--both monitors were immediately recognized, and easily configured. There were no shocks, no traumas, no dropped CPU, nothing that would be an obvious cause to the problem.

Thanks in advance for any help; I'll post here if I come up with a solution...
I've not had your issue exactly, BUT I've found DVI connection to be the most reliable with external monitors and OS X.  AS you said, you intended to try DVI-D next.  I imagine that would get both monitors firing up.

Side note: OS X 10.8 seems to despise 10.5.  I've had a bunch of unrelated madness crop up due to failed mountain lion install and then recovery via  at some point using 10.5 install disc.  Disastrous results.