Go to your control panel and click admin tools then computer management

From the left choose disk management; ok make sure your mp4 players is switched on and its showing the USB icon screen.

Can you see you’re your mass media mp4 player drive?
Does it say something like 24mb you may need to scroll down to see the drive,
If yes right click the panel and choose change drive letter an other small window will pop up press the change button to something other then one your already using..

You might want to try the format from here to right click and format

the format tool i use was from this site try looking in the download section...