I have a HD hooked up via USB and it mounts fine and I can view the root directory, but when I try to go into subdirectories, it says I don't have permission.

I tried doing 'chown -R username /dev/sdf2' but it doesn't work, saying that it is a read only file system. I tried to change permissions in nautilus, but I get the same error.

What am I doing wrong and how do I fix this?