Installing High Sierra on older PC

[sombrero]

I have an old PC with a Z390 motherboard and an NVIDIA GTX 1070. I was thinking about installing High Sierra because I don't want to spend money on a nostalgia box. I also have no interest in running newer versions of macOS, especially since they no longer support many older applications. Ideally, I'd run Leopard or Snow Leopard, but I don't have compatible hardware, and as I said, I don't want to spend money just for nostalgia.

I managed to put together a reasonably decent OpenCore configuration but the installation always fails. Then I found out elsewhere that the Recovery images downloaded under Windows are basically useless—they simply don't work because of disabled servers.
I also ran into an issue when trying to install High Sierra on a drive where the second partition is already occupied by Windows.

From what I've read, this seems to be a limitation of the Recovery installer only, which would make sense since Recovery is intended to restore the system from the original macOS installation image (so without any other OS).

 

So I installed Catalina in VMware and downloaded the full High Sierra installer from within macOS instead. Same story again: most of the servers and download commands for older installers no longer work, and when I finally managed to download the installer, the createinstallmedia command that was supposed to prepare the USB drive crashed after running for a while and eventually returned an error.

 

After several attempts, I finally managed to download the full installer to the Applications folder and create a bootable USB installer.

The USB drive is split into two partitions: a FAT32 EFI partition and an HFS+ partition containing the macOS installer (as an .app bundle) along with a few other files.

I copied my OpenCore configuration to the EFI partition—the same one that previously allowed me to boot Recovery. However, now when I select the macOS installer from the OpenCore boot picker, the PC simply reboots and boots into Windows.

It seems to me that OpenCore is unable to locate or launch the macOS installer. Previously the same EFI config worked fine with MacOS recovery image. I configured the EFI folder and config.plist under Windows, initially planning to use a Recovery image. After running into problems with Recovery, I downloaded the full macOS installer instead.

 

The problem is that the file layout is different between the two methods. When creating a Recovery USB under Windows, the EFI folder is placed on the same partition as the macOS Recovery DMG. With the full installer created under macOS, however, the EFI folder is on the FAT32 EFI partition, while the installer itself is located on a separate HFS+ partition.

 

Could this be the reason the installer won't boot? Is it possible that OpenCore simply has no idea where to find it?

 

Please help.

Edited 1 hour ago by sombrero
small changes in explanation