Well I'm in the middle of performing a hardmod on my 2015 n3ds xl. i've already built the sdcard cable device. Unfortunately, I ripped the ribbon of the lcd screen, and waiting for the spare part to come in the mail.
But as far as using 'dd', i'm not familiar with this, so I don't even know how to start. I was going to make a VM of a windows box, and forward the usb to it so i can use win32diskwriter but if i can do it natively on a mac, i'd rather do that.