I'd like to thank CaseySJ, the OpenCore project (and everyone else involved) for doing this work.
I built my box similar to the original setup, roughly a mid-level iMac, significant overkill for what I will use it for (web surfing, running a rails server for my billing system, occasional XCode and Rubymine usage), probably the last Intel-based Mac I will have:
Gigabyte Designare Z390 motherboard
i5-9600K
MSI RX580 4GB card
32GB RAM
Fenvi T919 Bluetooth/wifi card
2 Western Digital Blue 1Tb M.2 NVMe SSD's
Phanteks Enthoo Pro Tempered Glass Edition full size case
Corsair H60 water cooler
EVGA 750G+ power supply
Samsung 32" C32G55TQWN Monitor
I initially started out with the first page of posts in this thread, using Clover and installing Mojave (to be able to use QuickTime), and that worked ok, installed on the first SSD, then worked through the Dortania OpenCore installation process to install Big Sur 11.2 on the second SSD.
And this point, it was mostly working (a more complete setup of Mojave, a less complete (I/O wise) installed of Big Sur). I could boot into either setup. But I had to make sure to select the correct boot drive, as each EFI folder only worked with the OS installed on that drive. At this point, I realized mistake number 1, having 2 drives with the exact same model number w.r.t. the BIOS. I have to memorize the boot order of the drives as displayed by the BIOS. Then make sure to select the right OS in OpenCore/Clover. In hindsight, I would buy different size drives, so it's easy to tell which is which when booting up. (I don't really need the space.)
I tried upgrading the Clover EFI folder to OpenCore, unsuccessfully, typically generating a panic early in the boot process. It was nice to be able to boot off the other drive, to modify/work on the this efi folder (vs booting off a usb drive and working through cli tools).
Eventually, I came to the conclusion that the Mojave OS install was hooped (likely due to my not removing Clover bits from it correctly), wiped the drive, and reinstalled Mojave with the OC-066-Designare EFI folder [from this post:
https://www.tonymacx86.com/threads/...olt-3-i7-9700k-amd-rx-580.267551/post-2223836 ], with newly generated serial number/uuid bits, and like magic, it worked. And, I could also boot using that efi folder, and then into the BigSur OS on the other drive as well.
The only problem I had with this was the screen would flicker whenever anything was redrawn onscreen, in both Mojave and Big Sur. This was fixed by deleting "agdpmod=pikera" from the boot-args.
I then moved on to getting audio working. I have a Dell monitor soundbar, and it would play the chime during bootup, when the OpenCore OS selection screen was displayed (so the hardware was working), but audio didn't seem to be working within either OS. This also turned out to be a DFU error, I was selecting the different audio outputs (with the soundbar plugged into either the green port on the back, or the headphone jack on the case), and changing the volume level expecting to hear a system sound. Except....the "Play feedback when volume is changed" checkbox was unchecked in the Sound Effects tab... After a bunch of searches, reboots trying different alcd parameters, moving the jack back and forth, and different audio out port selections, I finally noticed this, and it was working all along.
Upgraded OpenCore to v0.6.7 release, and copied the EFI folder to the Big Sur EFI volume, and now both drives can boot into either OS.
The only outstanding issues/untested items for my setup are:
-front of case has 2 USB2 ports, which were originally hooked up to the motherboard, but were disconnected so the Fenvi card could be plugged in there. Probably will leave it like this, as there are two USB3 plugs right there as well that do work
-Thunderbolt ports are untested for Thunderbolt support. They work with a USB memory stick I have, but I only have a TB1 Caldigit dock, I don't have adapter cable/cables to hook it up, and it sounds like it needs a TB2/3 dock between it and the computer for it to work, and the computer has all the ports that the dock provides (which I use with my '13 MacbookPro 15" Retina computer)
-booting up, with the monitor connected via DisplayPort, it boots up a lower resolution mode (about 1280x800), and only switches to the native 2560x1440 mode midway through booting of Mojave/Big Sur. I tried setting the resolution in the config UEFI output panel, but it didn't make a difference. Also updated the monitor firmware to the latest version. When connected via HDMI, it boots up using the native resolution for the OpenCore OS selection screen, but I was getting some screen flashing like there was a poor cable connection. I'll probably go back to using a HDMI connection and see if I can figure out the problem there...
Edit, just to make this more complete:
I'm using firmware F9i, modified so CFG_Lock is unlocked for native NVRAM support.
I installed BigSur 11.2, and then updated to 11.2.2 via system preferences/software update. It completed, but I believe I had to manually select the correct update volume when rebooting for it to complete. In BigSur, the finder just shows the BS & Mojave volumes, but in Mojave, the finder shows Mojave, BS Data (with a notification about using unsupported features), and an Install volume on the BS drive. It is also listed in OpenCoreConfigurator, in the partition info text, but is no longer listed as a bootable volume when booting in the OC boot list (which just shows EFI/BS/BS Recovery/Mojave/Mojave Recovery and some efi tools.
