'As root I did a "chmod 777 /extra".'
You might try:
chmod 777 /extra/* -R
You also might move /extra to /home/user/extra, set the owner to user and set the group to whatever group all the users are in (for example rockers) with:
chown user:rockers /home/user/extra
chown user:rockers /home/user/extra/* -R
chmod 777 /home/user/extra/* -R
Then change /etc/fstab to make /home/user/extra the mount point and boot.
As another thought, your problem may be that your settings in /etc/fstab for /extra conflict with what you are trying to do.
Be prepared. Create a LifeBoat CD.