If you want to install macOS Monterey on your Mac and you Macs are not officially supported by Apple. I suggest you to follow this video.
I installed two of my Macs to macOS Monterey 12 beta 2 today, and I found for beginners that video was not enough. There were also something important that were missing. That was this article for.
Before installing the system, it is better to know how the booting process works.
Normal Boot With a Internal SSD
When a user presses the power button, a Mac searches all disk partitions. The EFI partitions normal are hidden and load first by default. So If a user chooses to boot from the internal SSD, the EFI are loaded first and then the APFS partition.
There are hardware related drivers on the EFI and this affects the macOS that loads after EFI. This is very important. So keep the EFI of you internal SSD and don't override them unless you are 100% certain.
Booting With Installing USB Disk
After a user presses the power button, the user hold the
Options button of the keyboard until a list shows. The list shows the Internal SSD, the EFI and macOS Monterey Installer on the USB Disk as well.
The user has to choose the EFI first then choose the macOS Monterey Installer. If the user chooses the Installer first, the system will show no disk error as the driver is not ready.
Installing macOS To External SSD
Installing to external SSD is as easy as to choose the external SSD. However, after installing, you should install the EFI by OpenCore Legacy Patcher to the external SSD as well.
Installing macOS on an unsupported Mac means installing both EFI and macOS on the Mac. Without the EFI, the Mac won't boot properly with the new macOS.
You will need at least 35GB free space to install macOS Monterey 12 beta 2.
For your USB disk, partition it in GUID format. It is request to use EFI with GUID.
For iMac 5K users, DO NOT install EFI on your internal SSD. Or your screen will only get resolution at 3840x2160 most, even on macOS Big Sur 11.4. That is because currently the EFI by third-party can't do as well as Apple's.