So let's take a quick look at a micro USB port shall we?
Alright so what the heck is this do these little 2 or 3 work letters mean Ava? Glad you asked lol.
1. VCC is the Positive supply voltage with a potential measured from GND in this case, 5V
2. D- is Used for transmitting data packets back and forward
3. D+ is Used for receiving data packets back and forward
4. ID is only really used in OTG cables to let them host device know it's host, the microUSB end, while the other device is slave.
5. GND or ground is the at point with zero voltage potential.
The need for two Data lines allows for full duplex of sending and receiving at the same time. Vcc is at 5V while the data lines have a max of 3.6V, again this is measured between that mine and GND.
Alright enough about that, what about my 3DS??
Well a lot of people that started using this would just chop up a standard cable, these do NOT use the ID pin, heck they aren't even connected. Knowing this you are left with VCC / D- / D+ / GND, well that 4 pins and the 3DS only needs 4 pins for the eMMC right? great....
Now here is the hazard, since people just chop up a standard USB cable that means that Vcc will be used on the eMMC, and well THAT is the problem, cause the eMMC is NOT 5v tolerant at most it can handle 3.6V as that is highest the pins can handle any more and it becomes destructive.
So my simple solution is why the heck don't we just use the ID pin and stop chopping cable but make our own? We are already buying female USB heads with all 5 pins, let's make use of it. Alright so what
I do is use ID for DAT0 on the eMMC, D+ for CMD, and D- for CLK, GND is still GND. Great that leave Vcc, the 5V potential outta the picture. But wait we can then use this to charge the system since the system takes in 4.5 to 5.5 V to charge, and we know this because while it says 4.7 on the charger a lot of stores sell 3DS USB charging cables.
Alright so the switch I like to add is two fold, it's so you can plug a normal cable in and it won't affect the 3DS or the computer, because it attaches the two USB data cables together while leaving the 3DS eMMC alone. flip the switch the other way and now you can plug in your SD adapter to read and write the eMMC.
But wait Ava! what if I accidentally plug in a normal cable but forget to switch it back to charge mode? Well don't forget and don't do that!! Buuut on the off chance that you do, I've done this before on purpose as a test with my N3DS, computer USB ports just error out and the 3DS errors out, unplug it and turn it back off then on and it should be fine, why is this? because the USB data lines do not exceed 3.6 V which means they will not be destructive towards the 3DS eMMD.