Make sure you have zadig if you're using Windows to enable the DFU driver. Instructions:
Many of the F7, F4 (REVO, ALIENFLIGHTF4, BLUEJAYF4, etc), and some F3 boards (SPRacingF3EVO, STM32DISCOVERY) utilise the STM32 Virtual Com Port (VCP) - a CDC serial implementation. This allows the UARTs on board to be utilised whilst the USB is connected. This requires the STM VCP driver to be installed so that the VCP to be recognised as an additional comm port on the PC. NOTE: this is similar to installing a USB serial driver, e.g. FTDI or SiLabs
The STM32 VCP driver can be downloaded here --> http://www.st.com/web/en/catalog/tools/PF257938
NOTE: Once you download and run the installation it has not installed the driver, merely unpacked the choice of drivers. Locate the installation directory and then run the EXE file pertaining to your system.
e.g. C:\Program Files (x86)\STMicroelectronics\Software\Virtual comport driver\Win8\ <- will have two files present. One for 64 bit systems (dpinst_amd64.exe) and one for 32 bit systems (dpinst_x86.exe).
in many cases, the above might not work. installing Virtual COM port drivers from SiLabs will solve the issues: https://www.silabs.com/products/development-tools/software/usb-to-uart-bridge-vcp-drivers