How to get iPhoto to index photos on the external drive

"You can create a second iPhoto Library and store it on the External. To do that hold down option when starting iPhoto and that will give you the option to creating the library, as well as the ability to switch which library is in use once you have more than one. The problem I suspect you will run into is that the photos will still be moved into the library and you don't want to share out a folder in the library to share with the windows app.

Aperture ($200) will let you create a library that references files outside of it's library. Aperture is geared towards a RAW workflow and still syncs with iWork and iPhone, but it is laking some iPhoto features such as Faces.

The application Shoebox, which is an iPhoto alternative will do what you want, but you of course loose some of the iPhoto specific functionality and tie-ins with Apple's other apps.

Edit: Actually, it is possible. Preferences --> Advanced --> uncheck Copy items to iPhoto library. You may still want two libraries though."