MP4 " formate error! " when adding new files

I purchase sony mp4player threw online shopping(rediffshopping), when i add more files in the palyer, at first i dont have inuf files to store in the player that time idont feel any thing, after adding converted amv formate files also no problem, after adding some more mp3 and converted amv files its saying "formate error!" there is no option to formate the player if done software corropts..how to solve the problem

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