Here's what works for me most of the time:
1.) Insert your USB Mp3-player to an USB slot, let it mount automatically.
2.) Check the System Monitor -> File Systems for the proper /dev address of your device (In my case this is /dev/sdc1)
3.) Open a normal Nautilus window and press the Eject button next to the usb device, which causes the device to unmount but stay in the system: If you open your "Computer" location from "Places", you'll see your device is still there.
4.) Open a root Nautilus:
~$ sudo nautilus
5.) Create a directory in your /media folder that will be your mount point for your player (I named mine "player")
6.) Mount your mp3-player with root access to the desired mount point:
~$ sudo mount /dev/sdc1 /media/player
7.) Now you can manage your files in the root Nautilus as you like.