I got the STM32F4DISCOVERY demo board at Embedded World last week. To my surprise the SWD (single wire debug) connector is not meant for external debugger. In fact it is the other way around. The board can be used as debugger to debug designs without on-board debugger.
I wanted to attach my debugger anyway. To disable the on-board debugger I hold it in reset by connecting it's reset signal to GND via SB10. To make it reversible I added jumper over SB10 solder jumper.
The external debugger can measure the VDD pin voltage for target detection. *
Single wire output (SWO) trace can be used when using the SWD. * NOTE: this is for trace only - not needed for (invasive) debug.
Even old style JTAG scan debugging is possible (it can be faster) if additional TDI signal is connected. It is at PA15. NOTE: SWO can't be used with JTAG.
Parallel trace on pins PE2 - PE6 is an option when JTAG is used * - not shown on photos.
* - if supported by external debugger
[Update] See how STM32F4DISCOVERY can be used as free debugger.