This article details the procedure for properly reinstalling OSX or downgrading from Mojave on newer Mac computers.
The following systems are manufactured with a new version of the management controller, called the “T2 chip”:
2018 MacBook Pro models
If you are uncertain whether or not your machine is affected by the upgraded management controller, you can check by searching for “System Report.” Under the section titled “hardware” T2 macs have an entry called “controller.” If there is no controller listed under hardware, the system does not have the new chip.
If a system contains a T2 chip, it will be necessary to follow an updated procedure for installing/reinstalling the operating system, otherwise permanent damage could occur.
If reinstalling the operating system on a T2 mac, it is absolutely necessary to disable the “Secure Boot” feature. If secure boot is enabled when the drive is erased, there will be no way to install a new OS and the system will be entirely unusable until the logic board is replaced.
To disable secure boot, please follow these steps carefully:
1: Turn on your mac while holding Command-R until the apple logo appears. This should enter recovery mode.
2: The “Utilities” window should display automatically once recovery has loaded. In the taskbar at the top of the screen, click “utilities” and then “startup security utility”
3: Enter your password if prompted.
You should now see the “Startup Security Utility” window shown here:
To complete an OS reinstall, it is necessary to select “no security.” It may also be necessary to set “External Boot” to “allow booting from external media” This will allow your system to boot to external devices, such as a flash drive if that is what is being used to load the OS. Once the OS install is completed, the previous steps can be followed to change the setting back to “full security.”
If downgrading from Mojave, it is necessary to ensure that the recovery partition is not erased. If the downgrade is corrupted or does not proceed as expected, the recovery partition is the only way to recover the device.
Additional information regarding upgrading your OS to Mojave can be found here: Mac OS Upgrades