I have a clever idea but it only works if:I have an iMac 11,1 (27" 2009) with a WX4150, I can successfully install Catalina 10.15.5 but not 10.15.6. The iMac firmware was updated with High Sierra in the past, but the patcher gives me the warning I have to update the firmware again. The system works flawlessly with 10.15.5 and boots APFS correctly.
Installing 10.15.6 completes the first files copy, then the system reboots as normal. But during the second install screen with the apple logo, the indicator touches the 40-50% then an error pops up saying the system is not compatible.
Actual Boot ROM Version: IM111.0039.B00 (is it the latest?)
I can't reinstall High Sierra to reflash the firmware as per apple rule, since the new GPU is not recognized and the old ATI 4850 is definitively dead, the OfficialAPFSFWUpdate.dmg linked into the Catalina Patcher installs, but no firmware updates occurs at reboot.
I don't have more ideas, so I'm asking for help. Thank you!
1. You own another High Sierra compatible Mac
2. You have either a FireWire 800 or Thunderbolt cable.
3. Your iMac is capable of entering Target Disk Mode
What you can do is put your iMac in Target Disk Mode. And use another Mac to complete the install.
Actually from what i recall GOP flashed cards cannot enter Target Disk Mode.
Actually here is a simpler idea. Create a High Sierra install to an external HDD/SSD (using another Mac) and then use either Carbon Copy Cloner to clone the disk over to the internal Partition. Disk utility might complete the clone as well, but it likes to complain alot.
Your iMac should be running firmware 215.0.0.0.0, does your current firmware support APFS booting?