I'm guessing you didn't explictly have to mount the device, i.e. it automatically mounted it when you connected it. In which case, it's probably an issue with how it's set up in /etc/fstab, so if you could list the contents of that file it'd help.
Also (and the eternal question)... when you say "it didn't work", could you be more specific? What error message did you get? If no error message came up, what made you think it didn't work? What user did you run the command as (if you don't have write permissions you almost certainly won't have the permission to change permissions)?