STM32F407G-DISC1 not working if not connected to pc
I experienced the same problem and there are two solutions to get the behavior of the old board.


Use the ST-Link Upgrade Utility to flash to an older version of the ST-Link v2/1. With version V2.J23.M9 it's ok. But you lose the compatibility to mbed. 
Open the solder bridge SB19. The new board has this bridge closed, resulting in a low BOOT1-pin. When the bridge is opened the BOOT1 is pulled to Vdd. If you do not want open the solder bridge, you can use an external jumper wire from Vdd to PB2.