in iPhoto preferences, you can choose whether or not you want to copy the files when adding to the iPhoto library, similar to itunes. If this option is checked, you will have a 2nd copy impedded in the iPhoto library folder (right click on the icon and select to "show package contents, you will see, by date, folders of the duplicate photos). I have never not had this option checked, as I too keep my photos on an external and know that if they are duplicated in the iPhoto library, they are all backed up at least one level. for sure, if this option is checked, you will have to delete from the finder. If the option is unchecked, I don't know. Just test it.