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It is y-cable.
Nintendo recommends having an external hard drive with an external power source. However i got a 50 dollar toshiba hard drive from best buy without one. In order to get something like this to work, you also need a y-cable to draw enough power from the system through 2 different usb ports instead of just the one.
You can buy a cable to do this, already https://www.amazon.com/Cable-Matters-Splitter-Y-Cable-Inches/dp/B00L1K1OIA
Quick follow up, is this a good one to get?
I bought this Y-cable for this external hard drive