How do I eject an external USB harddisk on Mac OS X? Well, if you were using it, it's well known and simple (drag the icon onto the trashbin).
However, what if there is no icon, e.g because there are no partitions on the harddisk known to OX X? Open the harddiskutilitiy - the external HD will be listed even if it cannot be used. Select it, then choose 'eject' from one of the menus.