EMC 2309 Can’t update to 10.13.6

I have been trying to get the late 2009 (EMC 2309) 3.06 GHz 27” iMac to normal boot macOS 10.13.6 High Sierra.

At this point, i can only boot 10.13.6 in safe mode.

The ATI Radeon HD 4670 GPU was toast. I sourced an OEM AMD Radeon HD 6770 512Mb pulled from a working mid 2011 iMac. Full functionality in macOS 10.12.6 Sierra.

On a clean install from a bootable USB 10.13.6 installer, the update loads onto the system. The problem occurs after the update installs and the system attempts a reboot. Grey screen with Apple logo and a progress bar that is 75% complete. Indicator (spinner) appears and hangs. System stays here indefinitely. Hard reset in normal boot mode brings the system right back to this point.

Hard reset and booting in safe mode allows High Sierra to load. I have reset SMC & PRAM, verified the disk, repaired permissions, and reinstalled. Still will only boot in safe mode.

Again, I sourced an OEM AMD Radeon HD 6970 1Gb GPU pulled from a working mid 2011 iMac. Full functionality in macOS 10.12.6 Sierra.

Still can only boot 10.13.6 in safe mode. A normal boot brings the system to the exact same stopping point as previously described (literally the exact same point). After weeks of forum reading, i’m thinking a possible kext (kernel extension) or config.plist file.

also, can’t access Apple’s hardware diagnostic tool. Pressing ‘D’ after chime does nothing. Only other mod is 4 x 4Gb OWC RAM. Booted in ‘verbose mode’ today to try and get some idea of where in the boot process it hangs, but, to be honest, i don’t know what it m looking at.

Any thoughts?

The AMD Radeon 6770 booted 10.12.6 normally on the EMC 2309 3.06 GHz . As does the AMD 6970 in the EMC 2309, but it does seem like there is a software/firmware conflict when trying to upgrade 10.13.6 on this system.