By default, an external usb disk is going to be available in /media and will only allow root to write to it. The quick way to allow access is to create a subdirectory and then give yourself ownership of the directory and you should be good to go.
So, I have an external drive I use for backups at /media/ext1tb. The ownership is root:root and permissions are rwxr-xr-x or 755. So, as my normal user, I can read, but not write. I then created a subdirectory called backup and gave myself ownership.
At this time, I'm not aware of how to change the ownership rules to allow me to own the actual device, but this should get you going.
sudo mkdir /media/ext1tb/backup
sudo chown username:groupname /media/ext1tb/backup