How to boot Mac OS X from USB media

* How to boot a Mac from USB media

Insert the USB boot media into an open USB slot.
Press the Power button to turn on your Mac (or Restart your Mac if it’s already on).
When you hear the startup chime, press and hold the Option key. Holding that key gives you access to OS X’s Startup Manager. Once the Startup Manager screen appears, release the Option key. The utility will look for any available drives that include bootable content.
Using either the pointer or arrow keys on the keyboard, select the USB drive you wish to boot from.
Once selected, either hit the Return key or double-click your selection. The machine will start to boot from the USB drive.

*Boot from USB: Windows

Insert the USB boot media into an open USB slot.
Press the Power button to turn on your Mac (or Restart your Mac if it’s already on).
When you hear the startup chime, press and hold the Option key. Holding that key gives you access to OS X’s Startup Manager. Once the Startup Manager screen appears, release the Option key. The utility will look for any available drives that include bootable content.
Using either the pointer or arrow keys on the keyboard, select the USB drive you wish to boot from.
Once selected, either hit the Return key or double-click your selection. The machine will start to boot from the USB drive.
*Boot from USB: Linux

Confirm the BIOS boot sequence lists the USB drive first, or make that change as needed.
After the USB flash drive is inserted into the USB port, press the Power button for your machine (or Restart if the computer is running).
The installer boot menu will load, where you will select Run Ubuntu from this USB.
Ubuntu will launch and you can begin to working in the system – setting preferences,
reconfiguring the system as needed, or running any diagnostic tools.