Apple has not released an Operating system that came on bootable media since it released OSX 10.7 in 2009. Apple has went to download-only installers, but if you find your self in need of a bootable installer your in luck. If you find yourself doing multiple installs, a USB drive is much faster than multiple downloads.
There are two options that will allow you to turn your downloaded 10.11 install into a bootable USB installer. There's the easy way with the graphical user interface and the advanced way that requires Terminal use.
This what you need:
- A Mac that you have administrator privileges
- An 8GB or larger USB flash drive
- The OS X 10.11 El Capitan installer from the Mac App Store in your Applications folder.
- If you want a GUI, you need the latest version of Diskmaker X app.
Create a USB Installer Using Diskmaker X App
Once you've downloaded Diskmaker X, connect the USB drive to your Mac and run the Diskmaker X app. If you get a warning from Gatekeeper about the app being from an unverified developer, right-click or control-click it and click Open to bypass. The app will offer to make installers for OS X 10.8, 10.9, and 10.10, but making it work with El Capitan is simple enough.
Diskmaker X has actually been around since the days of OS X 10.7 (it was previously known as Lion Diskmaker), and it's still the easiest GUI-based way to go without intimidating newbies. If you're comfortable with the command line, it's still possible to create a disk manually using a Terminal command, which we'll cover momentarily.
Select OS X 10.10 in Diskmaker X, and the app should automatically find the copy you've downloaded to your Applications folder. Since it's looking for a Yosemite installer, you'll need to click "select an install file" and then find your El Capitan installer in the Applications folder. It will then ask you where you want to copy the files—click "An 8GB USB thumb drive" if you have a single drive to use or "Another kind of disk" to use a partition on a larger drive or some other kind of external drive. Choose your disk (or partition) from the list that appears, verify that you'd like to have the disk (or partition) erased, and then wait for the files to copy over. The process is outlined in screenshots below.
Create a USB Installer Using Terminal
If you don't want to use Diskmaker X, Apple has a terminal command that can create an install disk for you. You must have the OSX 10.11 installer in your Applications folder and you have a Mac OS X Extended (Journaled)-formatted USB drive named "Untitled" mounted on the system, you can create a OSX 10.11 install drive by typing the following command into the Terminal.
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
The command will erase the disk and copy the install files over. Give it some time, and your volume will soon be loaded up with not just the OS X installer but also an external recovery partition that may come in handy if your hard drive dies and you're away from an Internet connection.
Whichever method you use, you should be able to boot from your new USB drive either by changing the default Startup Disk in System Preferences or by holding down the Option key at boot and selecting the drive. Once booted, you'll be able to install or upgrade El Capitan as you normally would.