I have a macbook pro with High Sierra installed. I want to install macOS Sierra on an external disk and keep getting the same problem.
I downloaded a copy on line via the Apple website. This tries to install it automatically but fails because I got High Sierra installed (which is what i want). I places a copy of Install MacOS Sierra.app in the applications folder.
I erase my external disk to the correct format and name it Sierra. I then use Terminal and the following command: sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/sierra --applicationpath /Applications/Install\ macOS\ Sierra.app
when this complete I restart my laptop selecting my external drive as the start disk.
It then starts to install Sierra but it fails with a screen with a message:
macOS could not be installed on you computer. Volume contains a macOS or OS X installation which may be damaged.
I have tried the whole process three times, including downing the installer app from apple, but i get the same outcome each time.
Has anybody any ideas or suggestion to overcome this