Thanks for reaching out to the Apple Support Communities! It sounds like you are having issues reinstalling OS X on your computer and I am more than happy to help! You have tried some really good steps so far and at this point, check out this article with alternate steps to follow for How to reinstall macOS.
For your second question about creating an external installer, I would check out this article with steps for How to create a bootable installer for macOS.
Cheers!
Dec 29, 2017 4:31 PM
Yes, upgrade from OS X El Capitan to macOS Mojave is absolutely possible. Check the below solution to upgrade from El Capitan to directly Mojave without facing any data loss problems.
New Apple MMGG2LL/A MacBook Air 13.3-Inch Laptop (1.6 GHz Intel Core i5, 8GB RAM, 256GB SSD, Mac OS X V10.11 El Capitan), Silver 4.6 out of 5 stars 54 Best Seller in Antivirus. System Requirements to Install Mac OS X El Capitan on VirtualBox: MacBook, MacBook Air.
Step 1: First Check your System Compatibility:-
First and foremost, make sure your Mac is compatible with macOS Mojave. In general, most Macs from 2012 or newer are compatible. Here's a list of Macs that can run macOS Mojave:-
1.1)MacBook (Early 2015 or newer)
1.2)MacBook Air (Mid 2012 or newer)
1.3)MacBook Pro (Mid 2012 or newer)
1.4)Mac mini (Late 2012 or newer)
1.5)iMac (Late 2012 or newer)
Macbook Os X El Capitan
1.6)iMac Pro (2017)
1.7)Mac Pro (Late 2013, plus mid-2010 and mid-2012 models with recommended Metal-capable GPU)
Step 2: Take Mac Data Backup
Before downloading any major update to your Mac operating system, it is very important to backup your data. The best way to ensure a smooth transition from an older operating system to Mojave is with Time Machine. If you don't already have a backup plan in place, there are a few options you can check out.
Step 3: Download and install macOS Mojave
Apple has made it very easier to simply download and install Mojave, even if you are running an older operating system. If your Mac is running El Capitan, Sierra, or High Sierra, here's how to download macOS Mojave.
3.1)First, click on the Apple icon in the upper left corner of your screen.
3.2)Click on App Store.
3.3)Click on the Featured.
3.4)Click on macOS Mojave in the Mac App Store.
Apple Os X El Capitan System Requirements
3.5)Click on Download under the Mojave icon.
3.6)Wait for macOS Mojave to finish downloading.
3.7)Click Install to begin the installation process.
After following the above steps, you will be able to upgrade your operating system from Mac OS X El Capitan to macOS Mojave.
Dec 29, 2018 1:17 AM