Has anything happened prior to the issue that you have now? Was the USB device unplugged by accident while transferring files? Have you tested the MP3 player on another computer? Does it work there or is it giving you the same error?
Please, navigate to the Disk management menu and try to check if the USB device has a drive lleter assigned to it. If not, you have to manually put a letter on it and format it. Beware that this procedure will erase all of your data stored on the MP3 player. If you need to recover the files, you have to search for a data recovery program.