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?

Thank you!


  • Can you reboot either while holding down the option key and then selecting the external HD? – At0mic May 15 at 23:15
  • Can't you, using your old MPB, do the same attempt at loading High Sierra onto an external drive? – jmh May 16 at 1:09
  • Because the external HD doesn't yet have a completely installed OS, it doesn't show up as an available boot volume when I hold down the option key (or in the Startup Disk pane in System Prefs). My efforts have failed, regardless of which MBP I use to try installing from a bootable USB installer onto an external drive. I also tried booting into internet recovery and installing directly from there onto an external drive, eliminating the bootable USB installer from the equation. Same result: when restarting it just boots to the OS on the internal drive and doesn't complete the installation. – dhnyny May 16 at 2:14
  • Please edit your question and provide the following information: 1. From where and how did you acquire the Install macOS High Sierra application bundle and what size is it? How, specifically, did you create the macOS USB Installer, did you follow the directions in How to create a bootable installer for macOS? Where and how did you partition and format the external drive for the macOS High Sierra install? How is it partitioned and what format did you use? Please introduce some paragraphs into, instead of a wall of text as your question. – user3439894 May 16 at 21:43
  • dhnyny, Thank you for updating your question with the information requested. +1 – user3439894 May 17 at 1:56

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