iPhoto stuck on updating thumbnails

Hi everyone,

I am running the latest version of iPhoto on Yosemite 10.10.3. On opening iPhoto, it gets stuck at 'Updating thumbnails'.

I brought up the repair menu by holding command+option -- the first and third repair options (repair permissions, repair database) and eventually get stuck on 'Updating thumbnail'. The fourth option 'Rebuild database' takes a long time to run, but successfully lets me into iPhoto with my pictures intact. When I close iPhoto and re-open it, the problem recurs. Opening Photos, it also hangs at 'preparing library, 14%'.

This is driving me crazy; I have spent 3 days on this. Please help!

Any advice is much appreciated 🙂