I plug my phone into my PC via a USB cable. It charges, but it will not allow me to access my phone via the USB connection. I search the forums and everyone says I should be able to pull down my notification bar and it will show the USB mode under "Ongoing". But it doesn't. It doesn't show that it is set for Camera, Charge Only, or anything else. And it doesn't give me an option to change the settings. I went through the settings menu, I have searched this board, and I have searched Google, but I can't find an answer. I am using the Galaxy S3, Version 4.1.2, no root or other changes. How do I change the USB settings so that I can access my phone files from my PC via a USB cable?