Boot from USB in macbook pro mid 2012 I have removed everything from my macbook pro. Now, how can choose USB as a startup disk? I tried using option    "SHIFT" keys also cmd+R. Nothing is working. How can I solve it? I am using Mid 2012 Macbook Pro and that usb is Ubuntu installer. [Btw, I am not interested to install Ubuntu in mac.]
The 'option' key at startup will give you the option of selecting another startup disk. The problem may be that your USB stick is not considered a valid startup option.