The purpose of this guide is to provide you with step-by-step instructions on how to unlock the bootloader on your Nvidia Shield.
Why Would I want to Unlock my Bootloader?
Unlocking your bootloader allows you to modify all aspects of your device to suite your needs which includes but is not limited to, installing custom Recovery images such as Clockworkmod Recovery or Team Win Recovery Project, installing custom ROM's, custom Kernels, as well as other MOD's and patches.
Will Unlocking my Bootloader Void my Warranty?
Warning: Unlocking your Nvidia Shield's bootloader will void the warranty, please note that even if you re-lock the bootloader the bootloader still show a flag that the device is voided for warranty.
This warning being given please also see this post by JordanKeyes regarding warranty information as it relates to hardware issues or hardware failures.
Unlocking your Bootloader
1. Backup everything on your device that you don't want to loose such any apps (And there respective data), pictures, videos, music, etc. as unlocking your devices bootloader will erase all data on the device. You can fully backup your devices contents without root using ADB's backup command or by using the Holo Backup Tool.
2. Download and install the ADB and Fastboot drivers for your Shield, the drivers can be downloaded here. You can also follow along with wwjoshdew's excellent video tutorial on how to properly install the ADB and Fastboot drivers for your Shield on Windows.
3. Make sure you have ADB and Fastboot setup and installed on your computer. If you haven't already setup and installed ADB and Fastboot a simple ADB and Fastboot installer can be downloaded from here.
4. Fully power off your Shield by long pressing on the Power button until the power menu appears, then select the "Power off" option.
5. Boot your Shield into Fastboot mode using one of the following ways:
5. A. Power on your Shield while simultaneously holding the Power button + Back button + Home button. Release the buttons once the Fastboot bootloader screen appears.
5. B. Run the following ADB command from a command prompt window (USB cable must already be connected from your Shield to your Computer):
adb reboot bootloader
7. Open up a command prompt window (cmd) in the directory where installed ADB and Fastboot to. For example if I installed ADB and Fastboot to a folder on my Desktop which is on my D: drive I would open a command window in "D:\Desktop\Fastboot_ADB".
8. From the command prompt windows enter the following command:
9. Now that your device is detected by Fastboot the following command is used to unlock your bootloader:
fastboot oem unlock
11. Once your bootloader has finished being unlocked, you will be taken back to the fastboot bootloader screen. From here you can power off your Shield.
Re-locking your Bootloader
To re-lock your bootloader you can following steps 4-8 of the procedure listed above and when you come to step 9 run the following command instead of "fastboot oem unlock":
fastboot oem lock
Now that you have successfully unlocked the bootloader on your Nvidia Shield you are free to flash custom Recoveries, ROM's, Kernel's, MOD's etc. enjoy .
[B]If you are stuck with the bootloader unlocking procedure I would recommend checking out this great video tutorial by wwjoshdew.