If you are using Win2k or WinXp, turn off the USB device on the system tray after the transfer, and then unplug it, otherwise the data may not have been actually written (similar to the meaning of saving in the buffer memory). Win98 system tray does not have this logo. You can try to do more operations on Mp3, such as sending things back, or unplugging them for a long time (during which the system does more disk reading and writing operations), which may solve this problem.