How to change monitor assignment in a dual monitor environment

It has to do with the number of graphics drivers and the way that those drivers enumerate attached displays.  The system starts with the driver for the embedded graphics ( i.e. Intel ), then proceeds to the discrete graphics, if present.  If there are no monitors attached to the embedded graphics then it's only dependent on the enumeration from the discrete graphics driver.  If you want a different order, play around with which monitor is physically attached to which port ( DVI1 vs DVI2 vs HDMI ).  Hope that helps.

Old thread but one without an answer to this day. A while back I was trying to figure this out and found no answer online. I believe I accidentally figured it out the other day.

It seems to be hardware/BIOS controlled and not software controlled. The entire process of what I did can be found here: HD7950 Dead after forgetting to plug in the PCIe 12v Rail?

Here is a simplified version that I think would work and not require you moving the GPU from one PCIe slot to the other and back. 

1. Shut down computer. Unplug PCIe power to Graphics card and monitor cables.
2. Turn on computer. Computer should refuse to boot and graphics card fan should be spinning at 100%.
3. Power it back off and disconnect all power.
4. Reset the BIOS using the motherboard reset jumper and pull the CMOS battery to completely reset the BIOS
   • If this doesn't work, move your card from one PCIe slot to the other and boot the computer. Then move it back to where you want it.
5. Reconnect everything except for the monitor that you want to be #1.
6. Once booted, connect the monitors one by one in the order you wish for them to be assigned.

I have a PC with Windows 7 on my old HDD and Windows 10 (upgrade from 8.1) on a SSD in a Bios selectable boot configuration.

Windows 7 assigns No 1 to my main monitor and No 2 to my old monitor.

Windows 10 assigns No 2 to my main monitor and No 1 to my old monitor.

I remember fiddling with Windows 7 three years back to set my main monitor to No 1 but I cant remember what I did. I need my high res monitor to be No 1 in Windows 10 to stop Adobe Lightroom crashes but so far no joy.

I suspect something has to be cleared out of the Registry but I am not comfortable with experimental hacking .

To be as clear as possible there is no registry setting will help you. Below is the only way proven to work. Sorry about that - it's terrible that the problem continues to Windows 10.

This is from my May 2014 post in this thread (words in brackets [ ] apply to your post).  You can do step 1 below on the existing cable that connects to your old monitor instead of buying an adapter (though I wouldn't do it if the cable is hardwired to your old monitor).

1. Buy a cheap M/F adapter and on the adapter disable the auto-detect pin - for VGA remove pin 12 with needlenose pliers, for DVI remove pin 16, for HDMI tape over pin 19. 
2. Unplug the TV [old monitor] so windows automatically makes your [new] monitor #1.
3. Reconnect the TV [old monitor] using the modified adapter and it should not take over the #1 assignment from the [new] monitor.

Found the resolution to this issue.

1. Leave the screen resolution screen up on the monitor you want listed as 1
2. Unplug all of your hdmi/dvi or other visual cables from the monitors or computers (whatever is easiest for you), excluding the one you want listed as display 1.
3. if you only have one monitor plugged in and still see a monitor listed in the display screen settings window, hit the detect button to flush it out.
4. From there, start connecting them as you'd like in order from 2 to #X.
5. Once done hit the apply button.
6. Previous to hitting the apply button my monitor 2 (right of my monitor 1) would require me to slide my boxes to the left of monitor 1.

I just had this issue after making a few changes

Steps 1-3 worked as normal, but step 4-6 didn't work for me (I have Win 7 home premium, Service pack 1).

The left monitor was assigned #1, the right was assigned #2.  I unplugged the cable for the left monitor and then the right monitor became #1 and everything (icons, taskbar, etc.) moved over to the right monitor as expected. I hit detect and identify and "1" showed on my right monitor as expected. I had to change from landscape to portrait to fit the orientation of my right monitor.

But when I plugged in the cable for my left monitor, everything went right back to the way it was before.  I mean literally everything - not just the left monitor being assigned #1 and being in landscape mode but every single icon went back to it's original position (I have about 200 icons semi-organized on the left monitor and 6 icons on the right monitor). This seems more than just a registry setting mentioned by someone previously.

I'd say Microsoft has been good at ignoring this particular problem but the icons going back to the way they were is new to me. A few years ago if the monitors went back to the previous number assignment all the icons would remain a jumbled mess in the old #1 monitor.  That was the main reason why I used the adapter trick for my KVM switch (and why so many people wanted to disable the auto-detect pin when using KVM switches). Before the adapter trick every time I used the KVM switch the icons would shift to the second monitor when it became #1 and then stay there when it became #2 again. 

4 years and counting and we're still waiting for a someone to verify a workaround. I'd assume someone new would try everything mentioned above from least effort to most effort and maybe something will work.  I'd say the adapter trick is the most effort (it's fairly easy though and costs about $7) but unless Microsoft sprinkled some new magic on Windows it's the one that works.

The auto-detect pin being disabled on a newly connected monitor prevents the #1 assignment from being reassigned to that new monitor which is the solution. If you have any doubts just search the web for  autodetect, pin, kvm.

I have a PC with Windows 7 on my old HDD and Windows 10 (upgrade from 8.1) on a SSD in a Bios selectable boot configuration.

Windows 7 assigns No 1 to my main monitor and No 2 to my old monitor.

Windows 10 assigns No 2 to my main monitor and No 1 to my old monitor.

I remember fiddling with Windows 7 three years back to set my main monitor to No 1 but I cant remember what I did. I need my high res monitor to be No 1 in Windows 10 to stop Adobe Lightroom crashes but so far no joy.

I suspect something has to be cleared out of the Registry but I am not comfortable with experimental hacking .