PX4FMU and PX4IO setup with APM

Ok, I was trying to setup my PX4IO board with the latest firmware.  This took way longer than it should, possibly because the instructions spread out on the various pixhawk/diydrones sites are outdated and rambling. Here are the steps that worked for me.

1. Disconnect the PX4IO from the PX4FMU, if it's already connected. I was never able to get this to work smoothly with the two already attached. 
2. Flash the standalone PX4FMU with the latest APM firmware. I used the qupgrade tool with the PX4FMU board connected via USB.
3. Place the latest "px4io.bin" file onto a microSD card, in the root directory.
4. Insert the microSD card into the PX4FMU. Remove the PX4FMU from any source of power (e.g. USB)
5. Attach the safety switch to the PX4IO.
6. While holding down the safety switch, attach external power to the PX4IO. In my case I used an old NiMH battery pack attached to the power input directly.  
7. The PX4IO should power up into bootloader mode. The red/amber LED will be pulsing rapidly,  the green light will be solid, and the blue light will be off as detailed here. IF the LEDs aren't lit properly to indicate bootloader mode, remove power and try again. 
8. Release the safety switch.
9. With the PX4IO board still powered, insert the expansion bus pins from the PX4IO board into the PX4FMU.  The good news here is that you don't need to hold down the safety switch anymore, once you get the PX4IO into bootloader mode. 
10. Since you've already flashed the PX4FMU with the APM firmware, the APM startup script will notice the px4io.bin file on the microSD card and install that on the PX4IO. 
11. To verify that the PX4IO updated properly, you can look for /APM/px4io_update.log on the microSD card. It should say something like "Loaded /fs/microsd/px4io.bin OK" at the end.
12. Note that you may need to reboot after this.  You can either use the reset button on the PX4FMU board, or use eg UART1 to issue the "reboot" command in nsh.

That's it...good luck!