For nearly two years, I used Lightroom as my external editor for iPhoto using the following process:
1. Select 'Edit in External Editor'
2. Select Add to Library (not Copy or Move) when the Lightroom import dialogue comes up so that LR is simply pointing to the file my iPhoto library
3. Make all my edits in Lightroom (my original is left untouched at this point, and LR stores data about how to process the original)
4. Export the photo from Lightroom into the same folder as the original using the same filename
At this point, I would get a prompt saying:
"The following files already exist:
path/to/file
Do you wish to overwrite the existing files, skip the existing files, or rename the exported files to avoid collision?"
I could simply choose 'Overwrite' and my file in iPhoto would be updated with the changes from LR.
This worked until right after Christmas. Then, in January, the process mysteriously stopped working for any new photos. Now, if I use the same process, I get the following warning:
"You can not choose to overwrite the existing files since some of them are the source files"