You’ve probably reset your Nexus One many times, simply by power cycling the phone or pulling the battery if the operating system freezes. This type of reset causes the hardware to reboot and the operating system to be loaded back into the phone’s memory. If you want to reset your Google Nexus One Android phone back to factory settings, you need to do a more complete hard reset, which removes all the applications installed in the device memory along with wiping your personal data from the internal device. Follow the steps provided here to reset your Nexus One to factory settings.
Press and hold the volume down button while simultaneously pressing the power button on the top of the Nexus One. When the menu appears on the screen, release both the volume down and power buttons. Use the volume down to navigate to the Clear Storage option displayed on the menu. Press the power button to confirm your selection, followed by pressing volume up to initiate clearing the storage on your Nexus One. The smartphone will reboot automatically when it finishes clearing data from your phone.
When the phone completes the reboot, you will be presented with the initial phone setup screen where you are asked to choose your preferred language. Keep in mind this reset does not change the contents of any microSD card you have in the phone’s card reader. You will need to reformat the microSD card separately if you want to wipe data from the card as well.