How to export iPhoto photos to hdd correctly

Hi what I'm doing is transfer around 5000 photos from my iPhoto on my mac to a hard drive on my windows 7 pc connected through a network via Ethernet around 8gb of the 62gb will transfer fine but then I'll get a error message saying cannot create volume/iPhoto back up

iPhoto back up is the name of the folder I'm transferring to on the pc

Any ideas why this isn't not working?

I read it could be because some photos are missing from the library but I've found and deleted all missing files and it still doesn't work!

Thanks