When I try to update my iPad from my Win 7 desktop, I get this error message which says
Could not start session with iPad and then the iPad disconnects.
Even during normal syncing I have noticed that my iPad disconnects a few seconds after doing the backup. It's for this reason that I have never been able to do a full sync of my iPad.
Note that the same usb cable/port works just fine for syncing/updating my iPhone 4.
I even tried reinstalling my Apple USB drivers as mentioned here but to no avail.