The flash chip is accessed via an eMMC controller.
SD Card Connections:
For the x2 GND wires use 12 AWG. For the other connections use 28 AWG.
2 = CMD
3 = GND
5 = CLK
6 = GND
7 = DAT0
Enable SMC_RESET (VERY IMPORTANT TO DO THIS FIRST!!) Place a 200-300 Ohm resistor between J4E1.1 and TP4E1
Disconnect the SMC clock from the Southbridge by removing R4D2. Save the resistor as it is needed for the Xbox One to function. If you do lose it then a solder bridge should work as the value is 0 Ohms.
NAND logic runs on the 1.8v rail and readers try to use 3.3v which is most of them don't work with the Xbox One NAND.
Bridge a 700-800 Ohm resistor between Pin 1 of U3C3 and Ground. This should make an output 3v3 on the rail without damaging anything because the SMC is held in reset, so it won't try to power anything on.
Directly underneath the NAND on the underside of the mobo are access points that will let you easily solder the CMD,DAT0 and CLK lines.
Everything works better when you hook everything up to the SD reader with everything turned off, then turn the Xbox One on and then plug in the SD reader to the PC.
Source: Team Xecuter / xpgamesaves