Make sure you have followed my Unroot Motorola xoom guide to make sure you have unrooted your tablet. Now, follow these steps to relock it:
Power off your device. Now, press and hold the volume down button while pressing the power button. This will boot the Xoom in fastboot mode. Connect it to your computer via USB and fire up command prompt. Type in this command:
fastboot oem lock
That is it. You have successfully relocked your Motorola Xoom
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