One thing I always do is make a bootable installer of any new Mac OS. In this case, High Sierra is the candidate. I used to create the installer on an external USB Flash Drive. However, these days, I just create it on a partition on my USB SSD External Tech Drive.
Install Disk Creator
In days gone by I have used the macOS Terminal app to create the bootable installer. Other times I used an app called DiskMakerX. However, I came across a neat little app on another Mac blog called Install Disk Creator by Macdaddy. This is a neat little app and it is free.
When you open Install Disk Creator you get a window that looks like this:
The App automatically looks for the macOS High Sierra in your Applications folder. That means you need to download the installer and then quit the install so the installer remains in your Applications folder. However, you could move that installer to another location on another drive and then navigate to and select it.
Once you have selected the High Sierra installer you have to select the device on which to install it. This is done by clicking on the drop down menu at the top of the window:
Don’t let this screenshot fool you. You see all those drives because I have my Tech Drive attached to my Mac. The Tech Drive contains several partitions of various macOS installers. In this case I installed High Sierra over my old Mountain Lion install. Don’t think I will be needing to install Mountain Lion too much anymore.
Once you have the source and destination setup, just click on “Create Installer”. The App gives you a warning that the destination device (whatever it is) will be erased. When you click OK it creates the installer on the device.
Now, there is one more thing which is pretty obvious. You should restart your Mac with the new installer connected. Hold down the Option ⌥ key at the startup chime until you see a list of your bootable devices:
I took this photo of the bootable devices on my Tech Drive. Yes, it is a little overkill. Here is the High Sierra installer:
It is important to start the installer running to make sure it works. You don’t have to actually install the OS right then, you just want to take the install process to a point where you are satisfied the newly created installer is functional.
You may not have a Tech HD like mine, but it is not a bad idea to make a USB Flash Drive installer of the macOS that you are using. The fact is, when you download a macOS installer it is placed in your Applications folder. Once the install is complete, the installer is removed. So it is nice to have a copy of it in an installable form. You may have to reinstall on your Mac sometime, or use it on other peoples machines.