how to fix ''error 003 unauthorized device has been detected''?

Sounds like wewe accidentally bought a hacked Wii. When wewe upgrade the firmware found the hack and now your Wii may be bricked. Try this: while holding down the reset button, turn on the power to the Wii. If a menu comes up then your wii is definitely hacked but it has a brick safety program to get wewe out of it. If nothing happens then wewe may be out of luck. Since wewe bought it from someone and it still allows wewe to play it then there is a good chance that your Wii is modded. I would say return it to the person au store wewe got it from but that seems to be out of the question.