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installing macOS Sierra on external disk

  rturner003 15:08 16 Sep 2018

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 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\ --volume /Volumes/sierra --applicationpath /Applications/Install\ macOS\ 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

  bremner 18:18 16 Sep 2018

Have the install on a usb drive.

Connect the usd drive and the external drive you want the install to be applied to.

Start the computer holding down the option key and when asked select the usb with the install.

This will boot and give you the choice of install destination.

  rturner003 18:26 16 Sep 2018

Thanks for the reply

Why a usb stick rather than an a thunderbolt external drive? All articles I read on the subject have said that you can use either

  bremner 20:30 16 Sep 2018

All that is important is the download from Apple must be on a different medium to the internal hard drive and intended destination drive.

  rturner003 20:44 16 Sep 2018

Could be but highly unlikely but if I get a few moments I may try it on a usb

  Pine Man 10:58 17 Sep 2018

You do not actually say what you want High Sierra on an external drive for.

Have you considered cloning your Mac Book to an external drive?

  rturner003 11:14 17 Sep 2018

It is not High Sierra that I want on it is Sierra the forerunner

  rturner003 15:37 17 Sep 2018

I have now tried to install macOS Sierra on a uSB Stick and I got the same outcome.

The date created on the file 'Install macOS' is 14th July 2017 created @ 21:58

So I am no further forward.

  BRYNIT 17:44 17 Sep 2018

A newer macOS OS will not allow you to install an older version even onto an external drive, the only way I can think is to boot from a bootable USB installer and select the external drive to install the OS. If you have the earlier OS CLICK HERE for instructions on creating the installer. CLICK HERE on how to install a clean install of sierra from the USB.

  rturner003 19:11 17 Sep 2018


you wrote the only way I can think is to boot from a bootable USB installer

I am correct in what you are saying is that I need to find someone with mac that has Sierra as it os and get that machine to download the install app and create the bootable USB. Would they need to complete the installation as well?


  BRYNIT 22:25 17 Sep 2018

I don't have a mac myself but due to my work I am having to learn how to wipe and reinstall the macOS myself as the person who originally did this has moved departments. It's a new challenge and I'm still learning.

What I understand from the links supplied is if you have a copy of the sierra OS or can down load it from the app store you should be able to create the bootable USB installer even from the newer OS. You then boot from this USB to install the OS onto the external drive. If you do not have the sierra OS you will need someone with mac with sierra on to download it for you. It would be easier and save time if they could install the OS onto the external drive for you as you would just need to boot from it when you get home.

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