How to allow MAC booting from external media

Hi BDAqua,

Thanks for reply but the process you have shared this process applies when mac has an OS already. But here in my case my macbook doesn't have any OS. And when I am trying to install macOS from the external media a error message appeared "Security settings do not allow this mac to use an external startup disk". To resolve this issue I used recovery mode where in the Utility option I tried to change but to do this an admin account and password is required. As my macbook pro doesn't have the OS. So it couldn't find the admin account.

The process listed by BDAqua would be correct for a new e.g. MacBook Pro Mid 2018 or MacBook Air Late 2018 which comes with a copy of the macOS already installed. If however you have erased the Mac so it has no OS then this will have erased the RecoveryHD volume as well. In this case you need to boot from Internet Recovery.

See - https://support.apple.com/en-gb/HT201314

It would be helpful to have confirmation of what exact Mac model you are trying to do this on, as an example no Mac Pro desktop computer enforces secure boot. (Yet.) Furthermore the Mac mini Late 2018 and MacBook Air Late 2018 cannot under any circumstances boot in to High Sierra. (The MacBook Pro Mid 2018 can but needs the special build of 10.13.6 for this model, there are ways to download this on another Mac using https://github.com/munki/macadmin-scripts/blob/master/installinstallmacos.py .)

1. Ideally you would use an Ethernet connection and then via that be connected to the Internet
2. You would then boot in to Internet Recovery Mode
3. You would then if needed reformat the drive using Disk Utility
4. You would then reinstall macOS
5. You would then let it boot in to the freshly reinstalled macOS, it will run the normal setup wizard and as part of this ask you to create the first user account, you need to do this
6. After completing the setup wizard and logging in reboot back in to (normal) Recovery mode - this time you don't need Internet Recovery
7. Once back in to RecoveryHD this time you can access the Secure Boot option
8. You can now turn off Secure Boot, it will ask you for the user name and password of the account created in step 5 above
9. You can then boot from an external USB drive

Hello Nasir,

Having a running OS off of an external drive is not usually recommended, so I strongly advice against it.

You can solve the absence of having an OS on your Mac by following the following steps.

1. Ensure your MacBook Pro is connected to a reliable wifi source that can hold up for about an hour.
2. Power off your MacBook Pro and then power it on.
3. As soon as the MacBook Pro is on and the background sound is heard, hold down the Command (⌘) – Option (⌥) – R combination of keys.
4. This will get you into internet recovery mode, which should be a grey logo of a globe.
5. After the internet recovery mode is all loaded up, click on the reinstall macOS High Sierra.
6. Clicking this should take you to a screen where you chose the disk you would like to install macOS High Sierra on. click on Macintosh HD at this point.
7. After completing the basic formalities of agreeing to the terms and conditions, installation of macOS High Sierra will begin.

The installation of macOS High Sierra is highly dependant on your internet speed. With 150 Mbps of internet speed mine took about half an hour.

When I reinstalled macOS, I got an error saying "Couldn't create preboot volume for APFS" if you encounter a similar error you can reply to this post.

Hope this was helpful!!!

Hey Nasir,

Have you erased your MacBook's ssd?

If you have done this, the security shouldn't give you a prompt, because there's no data on the system. I honestly have absolutely no clue why this is happening.

I'm really sorry I couldn't help.

To allow your Mac to use an external startup disk:

1. Open Startup Security Utility.
2. Select ”Allow booting from external media.” 
3. If you want to select an external startup disk before restarting your Mac, quit Startup Security Utility, then choose Apple () menu > Startup Disk.

* Mac computers that have the T2 chip don't support starting up from network volumes.

https://support.apple.com/en-us/HT208198