Flashing in Windows is not always reliable. If flashing in Windows doesn't work, then flash in DOS with MtkFlash.
When you run the official updater package from BTC, it should create a folder in your "WINDOWS\Temp" directory. If you look inside this folder, you should find a file named 0251.HEX.
Using 0251.HEX, you can flash your drive from DOS using MtkFlash. MtkFlash and instructions on its use can be found here: