Hello,,
I would like to share with you the solution I have used to solve the problem with Iphoto.
I hope it could help someone. I am sorry but I am not an expert.
I have an old Mac (2009). After migration to El Captan, the system became extremely slow and some app often frozen. After trying a lot of solution to repair El Captan I have decided the downgrade to Yosemite. Thanks to TM, not big problems.
I have OS backup, Media and Photo libraries on external HDs.
Time Machine has moved back the system to a date before the upgrade to El Captan.
Before to do this, I have saved all files and documents created or modified after upgrade.
After downgrade to Yosemite, it was quite easy to re-import on ITunes library only the music that was saved on external HD after the date of migration to El Captan.
The unpleasant surprise came from Photo (version Yosemite) that was not able to open the library created on El Captan.
The only solution seemed to import once more the photos in a new library loosing all albums, metadata, etc.
90 GB of memories well organized. Terrible.
This is the solution I have used. For sure not perfect and time consuming, but the best I was able to find. Condition to use this method is to have also the Iphoto library when migrated to Photo. You should have because El Captan di not delete or modify it.
On my external HD I have two libraries. The above mentioned Iphoto and the one I was using in El Captan. Now no more compatible.
These are the steps I have followed
1) Create a new Photo library in Yosemite.
2) Import in the new library the photos from folder master of Photo library (El Captan). Right click - show content – copy and paste the folder master. It takes quite a long time (I could import only folders 2015 and 2016 but I preferred to import all folders). After this I have in the new library all my photos but without any organization, album, etc
3) Open Iphoto and the old library before migration to Photo. In this case it is well organize with albums, metadata, etc. In particular I could see the date of the last photos imported in such library (in my case was 1st of May 2015). Close Iphoto.
4) Open the new Photo library and export in a new folder on desktop all the photos created after migration from Iphoto (in my case the 1st of May 2015). I have used export as original. Close Photo
5) Open once more Iphoto and import in the library the photo from the folder on desktop. In this way you have in old Iphoto library, all photos. Now you could re-organize only the last photos imported after migration (in my case from 1st of May 2015) and not a full library. Save the library
6) Delete the new Photo library created at point 1 (you just needed to take out from the not supported El Captan library the photos stored after migration from Iphoto).
7) right click on IPhoto library and open with Photo. Photo will import all photos including album, event, etc. it takes time but at the end you have a well organize Photo library in Yosemite. the IPhoto library wouldn't be modified.
Hope not confusing you too much. Bye