Hi Alpakkaya,
I just ran a quick search on this forum after experiencing this very same problem. However, the answer to fix this doesn't seem to be in this thread. How did you resolve this? With the 'export' from the 'file' menu? ... but what about when you have a mix of photos and videos in your event? does it work fine?
This problem occurred the first time on my mac while I was browsing photos. Suddenly all the thumbnails of my library turned black (with no previews). In a bit of a panic, I restarted Iphoto and it looked fine at first glance.
But later on, I realised that some photos were rotated in the wrong direction. More bizarrely some old photos I've deleted were back in some events.
It's from that day that I was unable to drag and drop whole events on the desktop or on external hard drives. Some events work fine but other are a real nightmare. Sometimes I can get away by copying entire rows of pix, but sometimes I literally have to drag and drop photos one by one to archive them.
I've tried all the options of re-building and repairing the library but nothing worked,
This is so annoying!!!
If you or anyone else have somehow found the cause of this problem. Please let me know.
Thanks in advance.
Tom