My phone “HTC 7 Mozart” was having problem in updating Windows 7.1 Mango “7.10 (7720)”. I kept on getting “C1010009” error whenever I tried to update.
After doing some researches and googling a bit, I came to know that it’s because of the USB port. I was using the USB ports which are in front of the system box . So, I switched my phone USB cable to the port at the back of my system box then it works.
If you are also having C1010009 error, please try switching different USB port and try updating again. Please feel free to share if you have any other solution!