Second, see this How To outline for creating a bootable El Capitan installer.
Using Diskmaker X is the easiest way to burn or make a bootable USB Flash drive. The OS X 10.10 Yosemite installer from the Mac App Store in your Applications folder An Administrator account on your Mac Method 1: Using DiskMaker X. First, review this introductory article: Create a bootable installer for macOS. An 8GB USB Flash Drive or larger where you would be burning the OSX Yosemite copy to. Bootable USB Installers for OS X Mavericks, Yosemite, El Capitan, and Sierra.The older method used to make bootable drives of OS X Lion and Mountain Lion installer did not work for OS X Mavericks. In this guide we will show you how to do just that. You’ll need a few thing to do this right. However, if you wish for a complete clean install of OS X’s latest on your Mac, the best way to go about this is to create a bootable USB drive of Yosemite. In the video below, I’ll show you how easy it is to create a bootable OS X Yosemite drive using software you can download for free.
I recommend making one for Yosemite, on an external hard drive or USB thumb drive, for many of the same reasons I recommend making a bootable Mavericks installer drive: If you want to install.