Wow, no idea.
If you start netxfer again (P key) does it always stop at this screen? The loading of frmware should only take about 3 mins and then the device will extract the image – you will see the details of the file copy on screen while this happens.
Try and P key some of the other problem units and see what happens.
Very strage as netXfer is quite reliable,
Cheers,
-TT