External Power STM32F4DISCOVERY
If it isn't working on the external power, try it with just the JTAG. Depending on your cable the JTAG could be supplying its power along with your supply which could have unintended results. 

Measure the Vcc voltage and make sure its set to what you intended (probably should be 3.3V)

This blog post describes how to program and set the jumpers.