Not able to flash recovery on my Shield Tablet K1 Important note: I used a Mac whilst doing this.

Also Important: I have the Android 6.0 update installed.

I got a Shield Tablet K1 for Christmas and attempted to root it. However, I'm having a lot of issues with it, and I'm worried I've messed up the recovery part of the tablet.

First, I unlocked the device, and set it up. This worked fine.

I went to flash the tablet with TWRP for the Shield Tablet, which I obtained from here: TWRP Shield Tablet

I used the command "fastboot flash recovery ", and it looked like it had completed.

However, it appears it has not completed.

Booting into Recovery Mode using "adb reboot recovery" brings me to a screen with the android lying on the floor, with a red triangle over him. 

Attempting to go into recovery mode using the down volume key &amp; the power button, usually brings me to the Nvidia boot screen, where the brightness increases after a few seconds but then it just sits there until it eventually reboots.

Then, after attempting to reflash the stock recovery.img that I got from here: Shield Open Source Drivers and Images it still gives me the a) red triangle and laying dead android or b) nvidia bootscreen that sits for a long time before rebooting.

I am completely lost. I can't tell why this isn't working - it should by all means be working, yet nothing is flashing.

If anyone can fix this, it will be much appreciated.
The shield's bootloader got overwritten during the Marshmallow OTA, and it is no longer backward-compatible (Lollipop ROM won't boot on this bootloader, and older recoveries neither).

The reason why you can't manage to boot into recovery is because the bootloader is preventing you from doing it.

When using adb reboot recovery, Android has already booted up, and, as a consequence, overwritten the recovery with the stock one. That brings you to the stock "android lying on the floor" recovery.

A developper at XDA made an EXPERIMENTAL Build for the shield tablet and shield tablet k1. (See here). Please use this at your own risk. Also note that Chainfire's SuperSU doesn't work with it.

If you want to root your tablet, you are not forced to do it by recovery. Give a this shot.