I'm having this exact problem - trying to export 20,000 photos from iphoto (my library is messed up and I want to start fresh). I just bought a new imac and restored from my backup of my old imac (which died), but iphoto appears to be messing it up - really, really slow performance, so my computer guy said I most likely have a corrupt iPhoto library and advised I export all my photos from iphoto and we'll start fresh. But every time I try (i've done the whole library and portions) it always stops after exporting all photos and says "unable to create/users/tuohy/desktop/iphoto *nameoffolder*"
I've tried creating the folder during the process, and then choosing one already created, but each time i get the same msg.
After I cancel the process 6,721 items are in the folder I created - so it has actually created the folder, just not copied all the pictures into it.
Any ideas? All i want to do is get all my pictures out of iphoto and onto a backup drive and then kill iPhoto and start fresh.