Is there a way to boot from a USB on a MacBook Pro with disc drive?
As soon as you hear the boot chime, hold the Option or Alt key.

Some icons should appear. Be patient if they don't appear immediately (but if after an entire minute the screen stays white there could be a problem). If it simply boots into OS X, you might have pressed the Option key too late.

There should be a hard drive icon for every partition on your hard disk and a USB drive icon for your external drive. Click on the desired boot medium and click the arrow under the icon or use the arrow keys and press enter after selecting the boot medium you want.