How to
Make A Bootable Install Disk for Yosemite on an 8GB USB Flash Drive
If you don't want to use Diskmaker X, Apple has actually included a terminal command that can create an install disk for you. Assuming that you have the OS X Yosemite 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 Yosemite install drive by typing (or copy-paste) the following command into the Terminal:
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.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 Yosemite as you normally would.
Read the complete article at ArsTechnica here .