We have discovered that the latest batch of Macbook Pro's we have received in from our supplier (2018 spec) no longer permit us to boot to external media and replace the factory install of Mojave (10.14.1) with a 10.13.4 disk image [that literally is just a out of box OSX not setup] which we have used loads of times previously (not the most up to date but its the best disk image i have ready to go before we thought we wouldn't use images!)
On booting the machine i connect my external hard drive and when trying to boot to my external OS partition or even OSX Installer I'm prompted to install an update from Apple to continue which when i complete i then get a prompt to say security prevents booting from external media - never heard of this for out of box machines but am aware its configurable in the OS once its set up...
If booting to the recovery partition and using disk utility i can replace the stock OS with my image but on rebooting after copying the data i get the NO ENTRY symbol indicating i cant boot the OS.
This is problematic as we havent tested Mojave yet so arent ready to release it to our users.
Anyone got any ideas?
To boot from an external drive on a T2 equipped MacBook Pro you will first need to adjust the security settings on the Mac. See Apple's About Startup Security Utility for info on how to do that. Note that to use this tool you must first have gone through Setup Assistant on the machine to create an admin user as you'll need an admin account to access the Startup Security Utility. Also note that you may need to select No Security rather than Medium Security to run the High Sierra installer.
And if that wasn't enough fun, if the 2018 MacBook Pro you received came with Mojave 10.14.1 or 10.14.2 you will need to downgrade to Mojave 10.14.0 before you'll be able to install High Sierra.
A general rule of thumb with Apple is that the OS that ships with the Mac is the lowest/minimum version that it supports. You'll probably find that it takes a lot of effort to even attempt to get the Mac to a lower OS, as there must be an admin user on the Mac before you can enable booting from external media from the Recovery partition.