Hello everyone
I just installed Fedora 42 (workstation) on an iMac 10.12.6 with an Intel Core 2 Duo E7600 x2 NVAC graphics card.
xrandr responds: Screen 0: minimum 16 x 16, current 1920 x 1080, maximum 32767 x 32767
LVDS-1 connected 1920 x 1080 + 0 + 0 (normal left inverted right x axis y axis) 480mm x 270mm
And to try as best I can:
03:00.0 VGA compatible controller: NVIDIA Corporation MCP7A [GeForce 9400] (rev b1)
My HDMI cable is plugged into a minidisplay adapter, since it doesn’t have an HDMI port. It goes to my 5.1 amplifier, which is connected to the TV. All this works very well on a Mac.
I hope I’ve given you enough information. I’m new to Linux, so I’m a total noob 
One last piece of information that might not be useful, but on the same computer, I tested Ubuntu. After installation, I got a black screen, which isn’t the case with Fedora.
If you have any ideas, I’m all ears 
Comments:
From the specs, I’m guessing that you have an iMac 21" Late 2009. That has an old Nvidia GPU - more on that later.
I’m also assuming that you have an external monitor attached to this system in addition to the built-in display of the iMac.
It looks like the iMac display is being detected, but not the external monitor. Is that correct?
Are you getting a graphical login on the iMac’s built-in monitor and nothing on the external monitor (because the monitor doesn’t seem to be detected)?
My suspicion is that this is related to the Linux video drivers. The Nvidia GPU in this iMac. is a GeForce 9 series chip - a very old GPU. It’s only supported by Fedora 42 with the “nouveau” third-party reverse-engineered open source driver that’s included in almost all Linux distributions. I’ve seen the nouveau driver have problems with dual monitor configurations with older Nvidia GPUs. I wonder if you’re running into something similar.
As far as I know there’s no other driver that you can try. The 340 series Nvidia driver (the last one that supported this GPU) was end-of-lifed by Nvidia long ago and doesn’t work with the Linux kernel version that ships with Fedora.
1 Like
Yes, that’s exactly it, my iMac is also connected to my TV with an HDMI cable (with minidisplayport, no HDMI port on iMac). Otherwise, on the Mac, indeed, it works. I’ll try to see with another distribution, even if I’ve already tested Ubuntu and I only have a black screen 
Thank you very much for this very complete answer.