I have recently bought the radeon 7870, which comes with 1 DVI, 2 Mini DP and 1 HDMI port.
My monitors are two VGA, one has a DVI cable which is my main monitor.
The other one has the original VGA cable going through a VGA>DVI adapter.
In my old setup I had 2 DVI ports and that worked, so I bought a XFX Active DVI>Mini DP Adapter and plugged it in.
CCC recognizes the other display, but my monitor doesn't appear to get any signal it's on standby and displays nothing.
I tried putting the active adapter straight on the DVI cabled monitor and then put on the cable with the VGA>DVI adapter.
Nothing seems to work even though extended desktop mode is on, I can see it on my CCC under displays.
The Mini displayport extended monitor just doesn't extend or duplicate at all.
I have tried reinstalling CCC, rebooted with just the mini DP cable plugged in. Nothing seems to work.
The monitor is detected but no signal is being given to display anything.
I am kinda missing my 2x DVI ports now, it's a bit baffling that newer generations are giving me more trouble connect monitors.
I'm at a loss now and am contemplating of sending this card back to get another one that supports 2x DVI.
I'm not going to get a new DVI/HDMI monitor just to have an extended desktop, this setup should in theory work.
Funnily enough I am having exactly the same problem with my Radeon 6850, and i have tried with DVI with 1 of my 24" and HDMI in the other 24" monitor.
I think it has the same ports aswell. I have also tried redownloading the drivers and nothing seems to work. I thought it was the combo of different cables so I have bought a new DVI cable with Mini DP adapter.
If anyone else can find a solution then i will be glad to hear it.
If your gfx card is anything like mine then the bottom DVI port is for 1 monitor only, it will not let you dual screen. If you use the top set of ports (DVI, HDMI, Mini DP) it should work, i did for me.
But i suggest getting an adapter for DVI to Mini DP as DVI gives a better output than HDMI (for computers) and VGA.