if you are using windows i have found that sometimes the usb ports u used for the phone get stuck to the driver that was there in the first place. So what I do to fix this is go into device manager from the control panel and then uninstall the drivers for the specific port that I am trying to use. While keeping my phone unplugged from the computer I then reinstall the samsung drivers, restart my computer and then try to plug my phone in. 90% of the time that has been what works for me.
Ok, I figured out the problem. It has nothing to do with the drivers or clearing ram memory. My problem is the actual micro usb connection. If is a little loose from side to side and when I push it to one side, it connects no problem.
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