HELP NEEDED - confused about iMac(2011) & GPU

[seefew]

Hi Folks,

I step on the spot and everything I've tried results in a no go

I hope someone have a hint in the right direction, what I could or must do.

My iMac is a Mid2011 (12,2), i7-2600, 32GB... and so on
The original GPU(HD6770M) is in working condition but I like to tinker with my Devices, and a bit more steam under the hood is always fine.
Now that I have juggled something with various cards, I am now completely at a loss.
I've tried with:
a Quadro K3000M with BIOS from MSI & DELL
a GTX 670M with BIOS from MSI
a GTX 770M with BIOS from XMG, MSI, DELL
a GTX 780M with BIOS from XMG, MSI, DELL

Of course there is no Bootscreen, but that's known.
However, with OS X Yosemite (10.10.5), the screen stays black with all these cards.
Tried it whit & w/o the Webdrivers.
For me completely incomprehensible:
If I start the iMac with a Debian SSD plugged in, the cards will work,
I get a picture on the integrated display, the mDP Port will detect a conectet TV

So what could i do to fix it?
Hope that someone can or will help me out.
Greets
seefew

 

[seefew]

No one in here who have an idea?

 

[bobfa100]

Hi Folks,

I step on the spot and everything I've tried results in a no go

I hope someone have a hint in the right direction, what I could or must do.

My iMac is a Mid2011 (12,2), i7-2600, 32GB... and so on
The original GPU(HD6770M) is in working condition but I like to tinker with my Devices, and a bit more steam under the hood is always fine.
Now that I have juggled something with various cards, I am now completely at a loss.
I've tried with:
a Quadro K3000M with BIOS from MSI & DELL
a GTX 670M with BIOS from MSI
a GTX 770M with BIOS from XMG, MSI, DELL
a GTX 780M with BIOS from XMG, MSI, DELL

Of course there is no Bootscreen, but that's known.
However, with OS X Yosemite (10.10.5), the screen stays black with all these cards.
Tried it whit & w/o the Webdrivers.
For me completely incomprehensible:
If I start the iMac with a Debian SSD plugged in, the cards will work,
I get a picture on the integrated display, the mDP Port will detect a conectet TV

So what could i do to fix it?
Hope that someone can or will help me out.
Greets
seefew

Upgrade your OS X 10.10 Yosemite to 10.13.4 macOS. Please backup your Mac before any upgrade.

 

[seefew]

thanks for your reply,
but I can't recognize why or how should an OS upgrade remedy the Issue?
especially since it has worked for several users with other versions of OSX already.