Installing High Sierra on an external hard drive

Thomi1975 wrote:

If I find a solution for this, using a USB hub for example, are you then suggesting to boot up the Macbook on the bootable drive and then select the external hard drive as the destination drive for the install? Will this work?

Yes, exactly.

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of High Sierra would have to be done on a computer that Qualifies to run High Sierra .

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is use a computer from Friend, Associate with a Qualifying computer that can run High Sierra

Assuming your Mac can run High Sierra…

How to create a bootable installer for macOS - Apple Support