Hidden Files

I transfer mp3 files from my computer to an 8gb sandisk micro sd card through a usb card reader.  The files transfer ok but when I remove the usb reader from the computer and put the card into my mp3 player it reads no files.  I put the reader back into the computer and no files show even though the space has been used so they are there.  I have ticked the show hidden files and folders in folder options.  It’s like the files are there but they’re invisible.  I have throughly formatted the card several times.  How do I fix this problem?  Thanks