Help with airport extreme

[aj casey]

Got an airport extreme,everything is fine untill I want to connect to a hard disk connected to the usb port on the unit.The airport is set to connect to the disk using a password when this is entered a message appears "Uknown user,incorrect password,or login is disabled.please retype the login imformation or contact the disk administrator" the password I am entering is correct as I have set it my self.I am usint the airport software and the airport disk manager apps.Any help would be greatly appreciated as I am about to throw a big wobbly with the darn thing