We gladly thank our user ARAKON for this tutorial.
If you need to get your 360 back working after a bad flash, you can finally do it.
When the 360 flash is bad (= system will not even turn on anymore), the system pulls up D1, 3 5 and 7 (3.3V).
This prevents the infectus from detecting the chip and from reading/writing to it anymore.
In order to fix that, the flash and chip will have to be supplied with external power, so the motherboard doesn't interfere with the flash read and write.
Infectus should look into that and ensure that the required powers are available from the chip itself when connected by USB, so people don't need an extra external power supply.
The 5V and GND points on the infectus have to be connected to a 5V supply and GND (GND on both the chip AND the 360 MB).. DISCONNECT 5V from the 360 MB!! (Infectus could take the 5V from USB).
Connect the circled 3.3V point in the attached picture to a 3.3V supply. (Infectus could provide 3.3V on one of the points). DO NOT CONNECT THE 360 POWER SUPPLY!
Turn on your power supply, then plug in the USB connector. The infectus will detect the flash as usual and you can write a valid firmware back to it. Be aware that you will have to repeat this procedure every time you have a bad flash. After you have a valid firmware on there, remove the 3.3V, 5V and GND wires and install like in the normal diagram.
Again, thanks to ARAKON for this useful tutorial.