Please refer to the posts below and let us know if it helps: -
Hey Sahil, thanks for the follow up. Unfortunately, it seems that the error is a widely used generic database error, and I could not find any sort of significant advice in the links you provided. BUT, it did reignite a fire under my *** to get my photos ported over.
It is currently functioning, and here is what changed. From a COPY of my database, I deleted all my photos after 2014. I then used the iPhoto import plugin and everything was working just fine.
I then overwrote that copy of the database with another copy of the full database. At the same time, I downloaded Aperture 3.6 from a link in the description of a YouTube video (since it is end of life there doesn't seem to be any other way to get a copy). I opened up the iPhoto library in Aperture and looked around for a few minutes. I then opened Lightroom and used the Aperture import plugin — rather than the iPhoto import plugin — and everything is now working fine. It's going on 12 hours and it's ~75% of the way through the import.
So, overall I am not sure what caused it to work. If I were to guess, I would say it was likely using the Aperture import plugin rather than iPhoto, or perhaps it was opening the database in Aperture (though that didn't seem to change the database at all. Regardless, it is currently working as intended.