Im confused which OS needs to be the same or above High sierra, is it the OS that is on your current ssd or the OS on your USB installer? So confused... Why would the installer on your usb need to be high sierra?
Although the thread I am about to refer to is biased towards Mac Pro computers, reading the 6+ year old section near the top of the page entitled "
macOS NVMe Support:" talks a bit about Sierra vs. High Sierra. It might/might not be relevant to your situation, I'm just pointing out info that MIGHT be helpful.
This is a general info thread for blade SSDs that can be used in the Mac Pro. This is a WikiPost so anyone with the proper credentials may edit it. NVMe SSDs can be used as a boot drive in the MP5,1 and MP6,1 with the latest firmware installed (beginning with version 140.0.0.0.0 for MP5,1 and...
forums.macrumors.com
As far as how your computer boots I will assume you are running OPCL (Open Core Legacy patcher) and here are some thoughts ...
When you first boot up, the firmware on your machine looks for a bootable UEFI partition. From what you have posted your firmware is up to date so it can find a bootable UEFI partition - if there is one - on your NVME SSD. But you are not there yet. If you are booting up from the installer stick, the firmware finds the UEFI partition on the USB stick (and I'm guessing temporarily loads it in memory) and passes control to it. The UEFI partition then (and I'm assuming stuff here) makes a list of the hardware available, and possibly the operating systems partitions available, and then (maybe with user intervention) passes control to the operating system. In the case of your installer stick this would be the very very basic OS available that is sufficient to host the OS X installation programme and some utilities and terminal programme.
I note that OPCL built the installer stick and (I'm again assuming here) AFTER a Macintosh aware UEFI partition was made, modified it in ways to work even better with non-Apple NVME SSDs. OPCL patched one of the UEFI partition OR the Installer partition (I'm not sure which) to trick the installer to run on a non-supported Mac.
Anyway, if Disk Utility can find the NVME SSD - which is why I suggested High Sierra at least for the installer - then you can format it (which will also create a small UEFI partition), and then install OS X on it. IF Disk Utility can't find it then you can try formatting it in a PC or a USB external NVME case. When you are successful you will have to boot from the UEFI partition on the USB stick AND THEN with user intervention, have control passed to OS X on an APFS volume on the NVME SSD. You fix this issue when you run OPCL to modify the UEFI partition on the NVME SSD. Hopefully this only happens the first time
I might point out that APFS is designed to work gently with SSDs, so I suggest using a Mac OS that uses and supports it.
I might not respond to this thread again. I think I'm spending too much time here trying to solve your problem. As I said, if you are successful please post what you did.
- Terry