help please.

I have tried and failed to transfer a 50 minute video from my iphone4 to my mac

the threads have suggested restarting the phone, having iphoto and tunes already open etc etc. Nothing works. shorter videos do still transfer no problem. the video is still on the camera roll.

I use a standard apple usb cable

the iphone does have 4.7GB of free space (I've deleted all music photos and videos so would have expected to see much more space - but that's all that is showing as available) I assume that the mac (0s -x 10.5.8 and iphoto09) has somewhere backed up the video on the first sync and now refuses to reaccess it

I vaguely recollect reading that there is a way to access backups, but cannot find out how