After plugging an external hard drive into the USB port behind the screen, can you upload MP3 audio files directly into the car or do you have to leave the external HD plugged in? If so, how? I have about 50GB of music that I put on a 1T hard drive that fits in there with room to spare and it was only about $100 so no biggie but I could use it for something else. Either way, the car is effing amazing. I came out of a 2011 GS and I am very impressed with this this Z51 3LT w/ auto.