Old Mac running OS X 10.6.8 with iPhoto 6 library containing 5,238 items
Imported data, including iPhoto Library, to new Mac running OS X 10.11.5 and Photos 1.5. via Migration Assistant.
Photos Library displayed only 1,794 photos (through September 2010), even though all photos were found in ~/Pictures/Photos Library.photoslibrary/Masters and ~/Pictures/Photos Library.photoslibrary/Previews.
Deleted Photo Library and iPhoto Library, manually copied iPhoto Library from old Mac again, relaunched both iPhoto and Photos - same result.
Attempting to run iPhoto Library Upgrader 1.1 (which "prepares libraries from iPhoto 7 or earlier so they can be upgraded with the current version of iPhoto") on yet another copy of the original iPhoto Library returned "Your library can open with the current version of iPhoto and does not need to be prepared with this tool."
Rebuilding the iPhoto library on both the old and new Mac (by holding down Command and Option while starting iPhoto, etc) did not help.
Opening yet another copy of the original iPhoto Library in iPhoto Library Manager showed only the same 1,794 photos and misidentified the library as an iPhoto 8 library.
Copied original iPhoto Library to a third Mac running OS X 10.6.8 and iPhoto 6 (all 5,238 items appeared in iPhoto).
Installed iPhoto Library Manager 3.8.6 (for Mac OS X 10.5.8 and iPhoto 7.1.5 and earlier).
iPhoto Library Manager.app > File > Rebuild Library. As Christoph Bartneck explains, "The software has a tool to recover corrupted libraries. But it works differently from Apple’s approach. It starts with a fresh clean library and imports all the photos, including their meta data, such as location, faces, date. It also manages to rebuild albums and folders."
Copied rebuilt iPhoto Library to new Mac running OS X 10.11.5.
Ran iPhoto Library Upgrader 1.1 (which now ran normally instead of returning "Your library can open with the current version of iPhoto and does not need to be prepared with this tool" as above).
Opened the upgraded iPhoto Library in iPhoto 9.6.1, which reported "This photo library needs to be upgraded to work with this version of iPhoto." After the upgrade process, all 5,238 items (including albums, metadata, etc) appeared intact!
Opened Photos.app which imported the entire iPhoto Library successfully.
Sadly, iPhoto Library Manager's successor PowerPhotos does not include a "Rebuild Library" command.
See also OSXPhotos, a "Python app to export pictures and associated metadata from Apple Photos on macOS."
/mac | Jul 18, 2016