I had this exact same problem, well one that was quite similar. My only difference was that I was switching between an old external drive (Firewire, 1T) to a new LaCie drive (Thunderbolt, 2T). So I wanted to move my iTunes and iPhoto to this new drive. The same message showed up in both instances (but the iTunes fix is a different story). Plus, the problems began to get worse
Suddenly, my iPhoto won't even open at all! It told me that my library had become unreadable and gave me some options to fix it. What I had to do is open up iPhoto while holding Alt and Command, then select: Rebuild Library. Even though this would fix that problem, I was afraid to do so because I did not have any of this data backed up. But in order to back it up, I would have to move it onto my LaCie, which didn't work in the first place. Following so far haha?
So I tried again to copy it onto LaCie-same error resulted. So I investigated; right click on the library and select show package contents. I was able to find where it stopped copying: at two files with the same name! (noted, one was capitalized). So I deleted one of them. Then I selected (back on the old drive) the rest of the iPhoto library immediately after the scandalous pair of files and continued to copy. I probably repeated this five times with five different instances. You might ask if it would have been easier to just go through by hand and pick out all the offenders, but the depth and complexity that iPhoto goes into sorting all your media was too much for me, and I'm lazy
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So eventually, my whole library is onto LaCie! Minus a few duplicates of course.
And then, as a side note, I felt safe rebuilding my library off of the old drive now I had a backup and everything is shipshape. Next I will chose to have iPhoto use the data on LaCie instead of the old drive and I will be all set. I hope this helps anyone with the same problems!
A Note: You remember how I mentioned that once you show the package contents on the iPhoto library that everything gets a bit deep and confusing? Well, from my experience all of you original photos that you ever put into iPhoto are in the folder labeled "masters." Even if you ever deleted them out of iPhoto, they are probably there. There is also a folder for "modified" and "originals." When I click on the original folder, however, it redirects me to the master folder. Just a little tidbit in case you were worried about a specific image of media file during this whole process.