If it is not named “Untitled,” you will need to change the command syntax for the pathname where it says: Volumes/Untitled.
How to make your external drive into a macOS High Sierra boot drive Your thumb drive or external hard drive is now ready. Click Done when the process is complete.
You can use a bootable drive on a hard drive partition for dual-software installation, to install on multiple Macs in your home, or as a bootable drive if you can’t use the Internet Recovery partition. When it comes time to upgrade your Mac operating system, the first thing to consider is whether you want to install it directly onto your Mac or create a bootable drive. Make a bootable copy of macOS High Sierra so you can have a portable copy to install on multiple devices.