Hi, wesley1!
Looks like You haven't mounted Your USB drive. When You connect it to the box, system detects it and prepares mount point for it (/mnt/sda1 in Your case?). So drive is detected, but unmounted yet, so its content is not accessible for the system.
To mount it You can type in the terminal window
sudo mount /dev/sda1
or if You use graphical desktop, start SystemTools->MountTool. If Your drive is red, means it is unmounted. Click on it, and it became green, now it is mounted.
Don't forget to unmount it after You finish Your task.
Have a nice Core!