Welcome to Android Central! How are you transferring the music to the SD? Via USB connection while the SD is still inserted in the phone?
Try unmounting the card (in Settings>Storage), removing it, and inserting it into the computer. Consider reformatting the card first, in case something got corrupted. Then use Windows Explorer to copy the music files directly to the SD card. Try copying them in relatively small batches, because some people have reported more problems when copying over a huge amount of data all at once. When you're done, put the card back into the phone and see if it works.