Talk:MacOS High Sierra/@comment-33909792-20171001001300/@comment-24886041-20171005154405

https://eclecticlight.co/2017/09/24/high-sierra-automatically-checks-efi-firmware-each-week/

In short, High Sierra checks your firmware against "known good" firmware of Apple computers. If your firmware isn't in the list (it's been hacked, it's not an Apple computer [Hackintosh], etc.) then you'll fail the check.

When this was introduced in the betas, some users reported the error being triggered when they attempted to boot from an external drive; I'm not sure if that got fixed or not.