Could be a long shot, and may not be of any help to you Pete, but go to:
iPhoto Menu ~> View ~> Hidden Photos
I once had a minor heart attack when a substantial amount of my photos suddenly disappeared.
This is a feature of iPhoto to help unclutter the library or folder view, you can choose to only show a selection of your best shots and just hide the rest.
To permanently 'unhide' you must select the photos once you enable the viewing of them as per above instructions, and press the keyboard combination of Command + L. A hidden photo has a large yellow X in the upper right hand side of the photo.
Another thing is hold down the Option key as you click on the iPhoto icon to launch it. A window appears inviting you to select an iPhoto Library or create a new one. Just check you haven't been accidentally working with a newer iPhoto library residing on your Mac's internal HD ... this would explain why your photos aren't showing up there ... I've been there and done this too
My photos reside on an Ext HD too.
I recommend you keep another back up of your photos, don't rely on the one Ext HD I would buy another Ext drive as soon as you can afford one for a backup of your backup.
I hope this helps ...