As the Mac is quite new I believe it has a T2 security chip (even though it doesn’t have a fingerprint reader which is generally the give away that a T1 or T2 chip is present on MacBooks).
The way the T1/2 security chips work is they are basically their own little computers and are the gatekeepers for much of the ancillary (I guess? They aren’t necessarily key ‘core’ components, but you need them to work the machine) go through the chip. Just off the top of my head I know the keyboard/mouse go through them, I believe they can do some hardware decode/encode so would make sense the graphics hardware is intertwined, and I think the SSD may go through it as well.... well, I guess that answers my previous comment: not just ancillary, but key core components interact via it as well.
The point is to provide increased security for the system & being an Apple product essential no documentation about how they work/how to interact with them is available. So basically a black box.
Given that FileVault is unlocked using it it makes sense that to boot a new Mac the security chip needs to be initialized and commands need to be issued to it in order to activate the devices that are connected via it. And as Apple is the only party with explicitly knowledge of how to do these things, thus only company issuing software that can do these things, getting an OS they don’t official support to boot is difficult.
However, it isn’t totally impossible either.
Quite a bit of effort has been spent trying to hack/reverse engineer the security chips and in fact a vulnerability (at least 1 major one) was discovered some time ago (the people that did so & those that wrote about it even tried contacting apple well before publishing anything about it, but seems as though Apple wasn’t/interesting in making a comment (let alone doing anything to plug the hole in people’s machines (if I recall correctly new hardware would be necessary, so replacing the chip with the very latest ones for apple silicone. If I recall correctly).
So this vulnerability has been using to boot foreign OS’, specifically Linux obvi, and there is some info out there on how to implement the ‘exploit’.
I don’t have the links off the top of my head, but armed with that info I’m sure it wouldn’t take long to come up with a search query that would get you what you’re looking for (bonus: I believe part of the name of the exploit was ‘rain’. Read about this a while back so...).
But be forewarned, at the time I read about it they were far from having full feature parity with the oldest models being the furthest along for obvious reasons. And there may be some cases, at least with MacBooks, where some of the hardware was unique enough that drivers would be needed from the manufacturer(s) who obvi don’t have any interest in making any.
As you’re Mac is a 2018 model & an iMac hopefully it will be doable, but definitely look into what they have working before hand (cause you could end up throwing a bunch of man hours down the drain).
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