When I reinstalled my iMac 24″, I restored a backup of the iPhoto Library.
But the software I’m working on (eXaPhoto Publisher) couldn’t import correctly an iPhoto album: a lot of pictures were missing. The imedia browser had the same problem, but iPhoto seems smart enough to find the pictures.
When you manage yourself your picture, iPhoto maintains a dummy structure inside the “iPhoto Library”. This structure contains links (aliases) to the original files.
I don’t know how it happened, but all the iPhoto aliases were marked as being on the backup volume. So instead of reimporting all my pictures and risk losing all my albums, I’ve written a Perl script to update all the aliases in “iPhoto Library/Originals” so they point now to the right location.