I'll try to keep this as simple as possible but it's a little complicated.
I am trying to install High Sierra on an external disk. The computer I'm using is running Mojave, however, so it won't run the High Sierra Installer.
I created a bootable USB installer for High Sierra. (I got the 5.2 GB Install macOS High Sierra application here: https://support.apple.com/en-us/HT208969. I created the bootable installer using the instructions here: https://support.apple.com/en-us/HT201372.)
Using Disk Utility, I created a Mac OS Extended (Journaled) partition on the external drive as my destination for the installation.
I rebooted with both the bootable USB installer and the external drive plugged into USB ports and selected the bootable USB installer as my startup drive. I then designated the partition on the external hard drive as my destination for the installation.
Everything seemed to go fine until the installer rebooted my computer. Instead of the completing the installation, my computer just booted up using the OS on the internal drive. When restart was complete, the external drive had a new folder called "macOS Install Data."
I tried multiple times, including pressing Option at the point of restart, but I cannot get the installation on the external drive to complete.
I imagine the "macOS Install Data" may contain the components that should be used to create High Sierra during restart but I can't get the installation to complete.
Can you help?