I photo library won't open it says it has already been migrated. All photos were not migrated.
When you cannot migrate an library again because of the error message "the library has already been migrated", you can repeat the migration, if you change the filename extension from ".migratedphotolibray" back to ".photolibrary". To get a better migration result the second time, make sure, that the iPhoto Library is on a volume with a compatible file system format, if you have been keeping your iPhoto Library on an external volume.