Greeting all!

Recently I encounted a strange problem (or it maybe very obvious for you guys) while copying files and I'd like to ask a question on file permission in linux.

I have a file, lets say 'biology.ppt' and it has permission as follows and i am the owner and it belongs to group 'root'

-rwx------

i want to change it to group, say, 'test', and give permissions to everyone to read and write and execute, what's wrong when I run the following command?

sudo chgrp test biology.ppt
sudo chmod 777 biology.ppt

nothing happened when i run these commands and i tried them with and without sudo. the file still has the permission -rwx------ and root is still the group.

if it matters, the file is on a usb disk mounted in /media

i also tried to create some file in my home directory and i can change it as i want with the same command.

any help would be greatly appreciated.