*EDIT: For factory reset ur phone u dont need flash nothing:
1. Backup ur data of ur Internal Memory
2. From your home screen press the apps icon in the lower right corner.
3. Find and press the settings icon.
4. In the upper end of the screen press the accounts tab.
5. in the bottom of the next screen press the Backup and reset button.
6. Press the factory data reset.
7. Read the info on your cell phone screen and hit Reset device.
8. Try to flash the firmware u wish again
9. If all go correct, copy ur data folders again in your phone and install the apps u had
I've did that before and it didn't work. I mentioned in the OP.
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