Greetings from Spain.
Just recently bought 2 chromecasts from local major electrical store in Spain both serial numbers starting 4127. Not sure of the stock levels but grabbed 2 as I am sure new stock will be arriving soon. Still about 8 left in store with this serial number.
Disconnected internet access to router by unplugging phone cable at the rear. Plugged CC in TV and used the android CC app to configure initial set up via wifi. Checked firmware on CC's and both showed firmware versions 15908.
Disconnected CC, used the Teensy ++2 board with the plusplus_ 16664.hex. Flashed CC using Hubcap.
Both perfect no issues whatsoever rooting went like a dream.
First CC- Just before starting the flash I reconnected the internet on the router to view the web page for the tutorial but luckily the flash process had started before my internet had connected otherwise the CC could have detected it and started the OTA firmware update ........ Phew ! didn't think about that at the time. CC not plugged into TV at time of root. Once finished connecting CC to TV and accessing web browser it already showed the firmware version 22062
Second CC - Made sure that the internet was disconnected throughout the process. On connecting the CC to the TV and accessing web browser to confirm root the CC was showing firmware version 17***** but shortly afterwards rebooted and installed the new 22062.
Remember to keep internet disconnected