Please go to the settings of the MP3 player and check for storage mode to be in MTP mode. Any other method used will give you problems. Seconds, please open Device Manager from Control Panel and check to see if there are any yellow exclamation marked items. If there is something related to the USB then you will need to visit the manufacturer of your device and download proper drivers.
On a side note, please check the total size of the content you want to transfer on the device because it is possible that you exceed the maximum amount of storage available that is available on the MP3 player.