I have exactly the same problem as in Cant open second hard disk "permission denied" except that my disk has ext4 filesystem. (don't have enough "reputation" to comment there)
To repeat the problem: I can only write as root.
The corresponding line of /etc/fstab is atm
UUID=<long stuff> /mountpoint ext4 defaults,rw,user,auto,exec 0 2
Linked answer doesn't work since
mount -t ext4 doesn't have
-o uid=<...>,gid=<...> option.