library-managementmacosphotosphotos.app
I imported a large number of old photos into iPhoto, and I then updated to the new Photos app recently. After the update, the thumbnails on the old photos are incorrect. They are just thumbnails of different pictures.
I cannot find a way to fix this. I tried launching Photos with CMD and Option held down and allowing the repair to complete. The problem persisted.
I have added an image that shows a thumbnail that doesn't match the selected photo. Any ideas?
Your Library is corrupted.
Download iPhoto Library Manager and use its rebuild function. (In Library Manager it's the FIle -> Rebuild command) Note: this does not require purchasing Library Manager
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.
I'd go for the 'not quite cave-man' approach - though still a bit manual...
Launch Image Capture.
Plug in iDevice
Select any/all photos you wish to delete
The Delete button is at the bottom of the window [the 'no-entry' sign]
After comments - the button is still there in Yosemite.
Update Nov 2015 - the button is still there in El Capitan, though it is now black & white, not red.
Picture updated to Image Capture 6.7 after further queries on the button not being there.
Update July 2016, as people are still missing this bit...
Best Answer
To Rebuild the Photos.app Thumbnails:
Backup your library before following these instructions.
Delete the old thumbnails and various parts from the library:
Quit the Photos app.
Open Finder, and go to the Photos.app Library, on my system it is called
Photos Libraryand is found in thePicturesfolder.Now right click the library and click
Show Package Contents.Next delete the folders called
PreviewsandThumbnails.Then delete the files called
ImageProxies.apdb,ImageProxies.apdb-wal,Library.apdbandLibrary.apdb-walfrom thedatabasefolder.Lastly delete the folders
RKCloudResource,RKImageProxyState,RKMasterandRKVersionfrom therecoveryfolder.Now rebuild the library
Open the Photos app while pressing ⌥+⌘. It will ask you to if you want to repair the library, click
Repair. It will do a repair, then your photos will go through a reprocess/reimport process. Once finished the thumbnails will start rebuilding.P.S. I tried this on my own Photos library, and it worked.