The Jelly Bean update changed the way that android structures it's files and consequently the way you can transfer files via USB. The new method is now MTP (Media Transfer Protocol) and will be the new "industry standard". All this means for you is that now your computer won't see your phone with the old drivers installed and you need to install new drivers for your phone so the computer sees it as an MTP device. If you google for "MTP drivers for windows! (or whatever OS your PC is running you should be able to find and install the correct ones, although windows should search and install automatically when you connect the device it does sometimes fail.....
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