Error transfering mp3 files

Question

after transfering about 30% of the songs onto my MP3 players, each and every player stops the transfer. media player didn't give an error message, but real player says unsupported format after using the exact same format for the first 30%. this is happening on my MP3 player and my phone when transfering music to it via USB cable. why is the format suddenly unsupported and how can I fix this?