adb backup also not working for me, disabled and re-enabled USB debugging, tried doing so with and without reboot, no change. Same issue as previously posted by someone else, the Shield does not ask for confirmation on screen to backup. Someone posted a solution somewhere to try enabling and disabling some developer desktop password, but the shield does not seem to have that option. A backup of stock would have been nice, but I am not overly concerned.
After following all the rooting steps and entering sudo fastboot oem unlock run for a few hours, and coming back to see no longer did my TV display a message 'please wait...', I entered the command fastboot boot twrp-220.127.116.11-shieldtv-unofficial.img but terminal just output: < waiting for device >, which it hangs on, and nothing changes on the TV screen. I tried to select continue or reboot from the Fastboot menu, in case that is necessary but missing from the guide, but something else was selected and a picture of an Android laying on his back with a hazard sign in its stomach appeared! Thankfully after cold rebooting the Shield did boot, although I had to run through the set up again.
I then followed the steps from this guide again, although this time when I enter sudo fastboot oem unlock it outputs:
...(bootloader) Bootloader is already unlocked.
[ 0.004s]finished. total time: 0.004s
I then entered the command fastboot boot twrp-18.104.22.168-shieldtv-unofficial.img and it again hangs on '< waiting for device >', with nothing changing on the screen, I just see the Cboot version, instructions on how to navigate, and the Fastboot menu: continue, boot recovery kernel, reboot, poweroff, forced recovery - am I supposed to select one of these before entering the command to boot the image?
If I kill the attempt with ctrl-c (Ubuntu 15.10) and enter adb devices it shows none are detected, although there were not long ago as I was able to boot into Fastboot in the first place with adb reboot. If I try the boot twrp command with a sudo before it the output is different:
FAILED (command write failed (Protocol error))
finished. total time: 0.000s
What can I do to boot the image please?