Whenever I turn my macbook pro on, I need my hard drive to start it up. How do change my start up disk to something other than my hard drive? I had the question mark folder and I reinstalled OS X and now I need my hard drive to turn it on.

Whenever I turn my macbook pro on, I need my hard drive to start it up. How do change my start up disk to something other than my hard drive? I had the question mark folder and I reinstalled OS X and now I need my hard drive to turn it on. I don't want to keep dragging my hard drive around whenever I want to use my laptop. I had to change my hard drive cable and it did not work, so I reinstalled to OS X. I do not like this, and I need help to change my start up.