I've changed the GPU (an old GT 640) with the Shappire Pulse 5500XT 8GB in my build with Gigabyte GA-Z87-HD3 and i5 4570. I have a working Catalina 10.15.4 (19E266) installation (generated with latest UniBeast 10.1.0 and MultiBeast 12.1.0. Clover is 5107).
Unfortunately, with the new GPU, I'm getting a blank screen at the end of the boot process (I've tested all DP and HDMI ports). I've tried with the various agdpmod=pikera, but without success. The system is up, but I cannot see any graphics (I'm able to shutdown the system with the usual keys sequence).
I decided to try a new installation of Catalina 10.15.4 to an external USB drive and it was completed correctly with the RX 5500 identified and running fine. I've then run MultiBeast 12.1.0 with the same configuration that was working fine in the previous installation (Clover UEFI Boot Mode, Virtual SMC v1.1.1, RTL8111 v2.2.2, AppleALC v1.4.7) adding only Core Graphics Fixup (WhateverGreen v1.3.7 + Lilu v1.4,2) and changing the iMac from 14,1 to 17,1. The boot options are: dart=0 agdpmod=pikera. But in this way it is not booting, stopping at the point showed in the photo.
However, if I boot using the installation USB key I'm able to reach the Finder (of course without any audio). I've tried with different combinations of settings (without AppleALC, only with Virtual SMC, etc.) , but without success. After two days of trails, I'm now without ideas on what to try/check and I'm asking for some help/hints/suggestions/advice from the experts on the forum.
The BIOS settings are: VT-D is disabled; XHCI Mode in Smart Auto; XHCI hand off is enabled; EHCI hand off is enabled; On board video card is disabled. Memory settings: profile 1 (not set, optional). Additional settings: Limit CPUID maximum: disabled; Intel Virtualization Technology: enabled. SATA mode: AHCI.
Many thanks.