Need help - iMac mid-2010 27" lost Target Display Mode after Flashing VBios for K1100m GPU

[tamnguyen28]

Hey everyone,


I’ve been using my 27" iMac (2010) as a second display for my Windows PC via Target Display Mode (TDM) without any issues. However, recently a friend used my iMac to flash the VBIOS on his Nvidia K1100m GPU. After that, I swapped my original AMD ATI HD 5670 512MB GPU back in, and since then, TDM has stopped working.


Now, when I connect the DisplayPort cable, the iMac shows a black screen for about 10 seconds before switching back to macOS — it no longer displays the Windows PC screen. Previously, it would only flash black for a second before instantly switching to the external input.


From what I’ve read, flashing the GPU can modify the VBIOS, and I’m guessing this might have affected TDM. I’d like to restore the original VBIOS on the ATI HD 5670, but I don’t have much experience with this kind of thing. I also haven’t found any clear guides or tutorials on how to do it, and I’m not even sure if restoring the VBIOS will fix TDM.


At this point, I’m stuck and unsure how to proceed. If anyone knows how to restore the original VBIOS or has any ideas on how to fix this, I’d be incredibly grateful.

I'm using MacOS High Sierra 10.13.6 which still supports TDM, and below are my display specs

 

[Bigwaff]

What version of macOS installed in iMac?
Please post screenshot or copy output of System Information > Displays.

 

[tamnguyen28]

What version of macOS installed in iMac?
Please post screenshot or copy output of System Information > Displays.

Hey Bigwaff, I have updated my post to provide the information that you required. Can you please help me check it?

 

[Bigwaff]

Have you tried the all the troubleshooting steps listed here?

Use your iMac as a display with target display mode - Apple Support

Target display mode is a feature available with certain older Mac models.

support.apple.com

I think GPU is fine. Perhaps issue seems to be resolution related. PC sending resolution not supported by TDM? Make sure PC send 2560x1440.

 

[Nguyen Duc Hieu]

Hey everyone,


I’ve been using my 27" iMac (2010) as a second display for my Windows PC via Target Display Mode (TDM) without any issues. However, recently a friend used my iMac to flash the VBIOS on his Nvidia K1100m GPU. After that, I swapped my original AMD ATI HD 5670 512MB GPU back in, and since then, TDM has stopped working.


Now, when I connect the DisplayPort cable, the iMac shows a black screen for about 10 seconds before switching back to macOS — it no longer displays the Windows PC screen. Previously, it would only flash black for a second before instantly switching to the external input.


From what I’ve read, flashing the GPU can modify the VBIOS, and I’m guessing this might have affected TDM. I’d like to restore the original VBIOS on the ATI HD 5670, but I don’t have much experience with this kind of thing. I also haven’t found any clear guides or tutorials on how to do it, and I’m not even sure if restoring the VBIOS will fix TDM.


At this point, I’m stuck and unsure how to proceed. If anyone knows how to restore the original VBIOS or has any ideas on how to fix this, I’d be incredibly grateful.

I'm using MacOS High Sierra 10.13.6 which still supports TDM, and below are my display specs
View attachment 2500594

View attachment 2500595View attachment 2500596

Your iMac looks fine for TDM.
How about trying PRAM reset?