When the player is connected to a computer, you are prompted to select [Charge & transfer] or [Charge & play]. 

Select [Charge & play] if you want to play files (eg, music) from the player while it is being charged via the computer.

Select [Charge & transfer] if you want to transfer files between the player and the computer while it is being charged.

If no option is selected, [Charge & transfer] applies.