When transfering many files, suddenly windows will make a noise and the current transfer cancels before it's done. Sometimes the phone disappears from the connected devices as if the USB cable had been unplugged. Sometimes it doesn't do that... it just stops the transfer as if it had completed successfully, but it didn't. Any ideas?
You can also try transferring the riles vie your hone network by setting up file and printer sharing in windows. You'll need a file manager on your phone that supports LAN transfers but thry are many available in google play. Give that a try and see if it works. You'll need to transfer the files to one of the public share folders in windows before you can share them via LAN.