KGB SDHC

I download mp3 music on my KGB SDHC, 10 to 15 gig, it takes more than 45 min or more, after the download my PC tells me that the correct amount of memory has been used on the KGB SDHC card but neither my PC or mp3 player see the music files, it is as if the files are there because they do take up the memory space but are invisible to PC & mp3 player and can not be used or played, please anyone help me