If it's an iRiver H10 model like mine there's a trick to make it recognise as a removable USB drive. Unplug the player from the computer and remove the battery. Push the battery back in then connect the player to the computer. Hold down the o button then turn the player on. Keep the o button held down till it says "Emergency Connect" in the top right corner of the screen and release the button. It should then get recognised as a removable USB drive by the PC and you can just drag and drop files onto it. This might also work on other models, but I have only tried it on the H10 model.