If your USB thumb drive, USB-based external disk or flash storage contains a usable operating system or an OS X installer, you can select it at boot time using OS X’S built-in feature called Startup Manager, which can be invoked via a simple keystroke.
Step 1: Turn on your Mac by pressing the power button by choosing Restart in the Apple menu, or restart it if it’s already on.
Step 2: Press and hold the Option (⌥) key immediately upon hearing the startup chime. Release the key after Startup Manager appears. Startup Manager will scan and list connected drives and volumes that can be booted from, as shown below.
Step 3: Highlight the volume you want to use by using your mouse or trackpad, or left and right arrow keys on the keyboard.
Step 4: Double-click or press the Return key to boot your Mac from the selected volume.