Was updating to Wildcat 4.5.
Normally I have had no problems doing this from my main machine running Windows 7 even though I have HW 2.72 and an old DFU3.11C
My main machine is on Windows 10 now and I expected that not to work as the DFU driver is not compatible with the older loader firmware and I there is no equivalent of DFU3.45 for my hardware.
My back up plan was to use a laptop still running Windows 7. It recognised the disk DFU OK but repeated copy attempts attempts using Explorer failed with error messages around disc properties changed.
It looked like Explorer was fiddling with the disc and somehow causing a problem.
I then tried opening a cmd windows and just used copy app1.hex e:\app1.hex (e was the DFU drive) and that worked no problem and firmware updated successfully.
So for anybody struggling updating then it might be worth trying a simple copy command instead.