For all those people who had issues with their new 820's, it appears to be due to the SD card. Windows Phone 8 is a completely different OS which can handle only a limited file types. Hence, please try the following, and you should get success for sure. Whether its a new SD card or from your previous phone, same solution is applicable.
1) Move all the media contents (photos, music, videos, sounds & documents) from the card to a PC.
2) Move any other necessary files to another folder on PC.
3) Format the SD Card using ur Lumia.
4) After formatting is complete, then you can move all the media stuff back into their respective folders.
5) Plz keep in mind not to move any other file types apart from the media files & office documents.
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