Here is the line inside my /etc/fstab for the neuros
/dev/sda1 /mnt/neuros vfat users,user,noauto,umask=0000 0 0
When I umount /mnt/neuros it will stop and wait, (as linux buffers disk writes) so it will take a little while to transfer the files from the buffer to the actual unit. I just wait until the neuros says it is safe (generally 5-10 minutes when I transfer around 30 songs).
The sync option above will make it happen in realtime I believe.
Try this to sync
/dev/sda1 /mnt/neuros vfat users,user,noauto,umask=0000,sync 0 0