Create a High Sierra Bootable USB Install Disk w/ Mojave

grevans Said:

“Am I using the correct Terminal commands to make this Bootable Install Disk?”

I think this has what you are looking for:

How to Create and Use a Bootable Installer for macOS - Apple Support

If your MBAir has had OSX 10.12.4+ installed, have you tried booting into Internet Recovery Mode using Command + Option + R to install the latest supported version of macOS for this system. This does not work with all computers, but should work with most of them.

Ok so first of all lets wipe that USB stick, I take it it is at least 16GBs in size. open Disk Utility, click on View in the menubar and select Show All Devices, now go to the left hand column and select the USB Disk not the indented Volume now click on Eras and give the Disk a name, set the Format and the Scheme as shown in the screenshot below. Once the the reformat has completed, move the Install macOS High Sierra.app to your Desktop. Control-click on the install app and click Show Package Contents, click on Contents and then Resources. Now open Terminal and type in sudo. followed by a space. In the Resources folder you will see createinstallmedia, drag that into the Terminal window (the full path of its location is now pasted into Terminal, beats all that typing) now type in --volume (and then a space) now drag the USB disk image on the Desktop into Terminal, again the full path will be pasted in. Now press Return and follow the Terminal instructions as before. Do not use the Mac for anything else whilst it is doing this.

Once the process is completed then you can eject the USB and put it in your other Mac, boot up from the USB and see if you can install High Sierra. If you are still having problems then it may be a problem with the USB itself, try another, minimum 16GBs. If that does not work then try downloading the High Sierra install app again (you will need to delete the original one)

and create another USB installer, do not do anything else with the Mac whilst it is downloading.

There is a free app that does all the work for you, https://diskmakerx.com/diskmaker-x-for-macos-10-13-high-sierra-is-ready/

if you want to try that.

Do you know why your Mac crashed in the first place, its seems a bit of an over reaction to re install the OS if it only crashed, tell us more about your problem.

As has been already mentioned, possible the downloaded HS is corrupted or also mentioned USB issues.

For me, I just did a HS USB installer yesterday from within Mojave and all went fine. Used the following Terminal command

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ High\ Sierra.app --nointeraction