Originally Posted by LordBli
Try to reinstall all drivers by connecting your phone and install Sony pc compagnon... Maybe it'll work !
Just give it a try !
Sent from my C6833 using xda app-developers app
Thanks. I was able to get into fastboot mode and install the kernel, finally! Now I have root on my Ultra with the new KitKat .681 firmware. Still not sure what the problem was. I suspect it was my desktop PC. It runs Windows XP. I switched to my laptop that runs Windows Vista and just re-installed PC companion and reinstalled Flashtool and the drivers from scratch and everything worked like a charm. Happy to be rooted on KitKat 4.4.2!
Device: Sony Xperia Z Ultra C6802 Android 4.4.2 firmware .681.