I have a 2018 MBP running High Sierra. I need to do some troubleshooting on another older Mac that is running Sierra, so I want to create a bootable USB key of Sierra. Unfortunately my High Sierra MBP won't allow me to run the Sierra installer, saying:

This copy of the “Install macOS” application is too old to be opened on this version of macOS.

Is there any way to create a Sierra bootable USB key, from a machine running High Sierra?

  • what command are you issuing to create the USB disk – Ruskes Dec 29 '18 at 1:53
  • None, I just want to run the Sierra installer and choose the USB drive as the target for installing onto. The disk is formatted Extended Journaled with GUID scheme. – Jonathan van Clute Dec 29 '18 at 1:55
  • So you downloaded the Sierra and used sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app to create bootable Sierra USB disk. If your drive is named something else, you need to change Untitled to the name of your drive. – Ruskes Dec 29 '18 at 1:58
  • No, I am not doing anything in terminal. I'm not looking to make a bootable installer, which I think is what you're talking about doing. I want an actual bootable drive running Sierra so that I can run other apps from it. – Jonathan van Clute Dec 29 '18 at 1:59
  • The above command will take the Sierra that you downloaded to your High Sierra Mac, and it will create Sierra USB. Now you can use that USB and run your Sierra Mac from it. – Ruskes Dec 29 '18 at 2:03

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