Can't save new files onto my mp3 player

Question

I have a JNB SSF-F320Bl mp3 player.
When I plug my USB port into my computer I can read the files on my mp3 player but can't save anything new onto it, it says "disk is write protected". I also can't organise files into folders for the same reason. It seems like sometimes my computer can read what's on the mp3 player, and sometimes it can't. Likewise, sometimes it can save new files, sometimes it can't??