Try to reconnect the USB drive to your Mac. Then, check if the missing music files are showing up on the USB drive. If not, move them again to your USB.
If this doesn't work, empty the Trash on your Mac. Maybe you ignore that deleting files from the USB won't release the space these files occupy until you empty the Trash. In technical, the deleted files are still stored on the USB drive and are located in the Trashes folder there.
So, although your USB drive has 256GB of capacity, if you use it over the years and never empty the Trash, it is inevitable that your Mac will say the USB drive is full but it isn't.
Also, your USB drive may have some errors, so you face such trouble. Go to Disk Utility to run First Aid to check and repair it. If First Aid can't make it back to normal for adding the music files, you have to reformat it and then move the files again.
Hope my tips can give you some help. Best wishes.
