By default, the gvfs/udisks2 stack requires a root/superuser password when mounting internal partitions. Setting the “user” option in your /etc/fstab file will not change this behavior in the gui (by default, Thunar) file managers that utilize gvfs and/or udisks2.
Authentication for udisks2 is handled with policy kit. If you wish to override the default behavior of requiring a root/superuser password, then paste the contents below into a file called /etc/polkit-1/localauthority/50-local.d/50-udisks.pkla to allow any user in the “users” group to mount partitions without a root/superuser password. Changes should take place immediately. Note that this will NOT override the filesystem permissions.
Content of new file /etc/polkit-1/localauthority/50-local.d/50-udisks.pkla