I have a 30" ACD and a 24" Dell hooked up to my Mac Pro, and most of the time I find myself only using the ACD. I normally keep the Dell powered off, but SL still detects its presence 
This actually creates a problem, because I invariably lose the cursor every now and then when my mouse strays off to the phantom display's side.
I don't want to have to keep unplugging the video cable on the second display to make it go away, and I don't want to keep it on constantly, only so I can know where the cursor is at all times. BTW, mirroring does not work, because the resolution on the 30" is reset to 1920x1200 (i.e., the lower of the two displays). I know I could get a KVM switch, but I'm hoping there is some setting in OS X (even if only accessible through Terminal) that can stop my Mac from detecting a display that is actually turned off.
This actually creates a problem, because I invariably lose the cursor every now and then when my mouse strays off to the phantom display's side.I don't want to have to keep unplugging the video cable on the second display to make it go away, and I don't want to keep it on constantly, only so I can know where the cursor is at all times. BTW, mirroring does not work, because the resolution on the 30" is reset to 1920x1200 (i.e., the lower of the two displays). I know I could get a KVM switch, but I'm hoping there is some setting in OS X (even if only accessible through Terminal) that can stop my Mac from detecting a display that is actually turned off.