Setting up a second monitor so both are the same (mirrored)

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I've been using a second monitor as a big screen to show detail when doing courses or watching vids. (it is on the wall behind the imac pointing the other way.
I added a second outboard monitor (above the imac) so I could get more screen space while I am working on a project.
This worked well, but now, suddenly it seems that it will only show the screen on the large monitor so I cannot control the computer from the imac itself. The screen completely transfers to the big screen (space 17). Additionally the display Sys pref does not show the setting available like "alignment" or "mirror display". I have upgraded to 12.6.9 .
 
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flyboyy
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The thread about cables is not apropos, as I have the apropos cables and have used them for a long time. It is just that lately there is something unusual going on that started not long after I started using the one additional monitor above the imac. Now the sys prefs/Display seems to have changed as per original posting. I downloaded and installed the latest version of MacOS 12.6.9
 
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When I got my M1 iMac shortly after they came out, I was able to run my karaoke program (kJams) with two mirrored TV sets displaying the lyrics. A few months later, I updated MacOS and that ability went away. The Apple Geniuses showed me that multiple external monitors were not supported by my computer (despite the fact that they *did* work before that upgrade).

So I had to do some research. What I needed was an adaptor that used "Display Link". So I bought https://www.amazon.com/dp/B09RMVL865/?tag=macforums0e4-20 and then downloaded DisplayLink How to install DisplayLink Software – DisplayLink Support .

That solved my issue.
 
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flyboyy
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Thanks Howard, Excellent post!
I started a reply to your post and then got sidetracked and didn't complete it.
I called up Applecare this AM and we worked on the issue. Somehow we ended up getting exactly what I wanted in that the main iMac screen is working and the screen above is available to drag things back and forth and the third screen is a mirror of the iMac itself.
One cable seemed to work better in the other slot. (Thunderbolt)
 
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flyboyy
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The plot thickens. I think I have finally figured out the problem. I was using a mini display port cable instead of a Thunderbolt. Sometimes it would work and sometimes not... I can relate to that.
 

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