I had a similar problem trying to get a user to be able to read and write a vfat partition.

As root, I chmod 777 /mnt/hdx but as the user, the icon wouldn't allow access.

Out of frustration, or lack of a better solution, As user I simply right clicked on the desktop icon for the partiton, clicked on the device tab, clicked on the arrow (down), chose the partition and everything worked. Worth a try..