If your GC can't read miniDVDs correctly (Disk Read Error) read this guide to solve the problem.
Open your GameCube and remove the laser. You can see the laser is holder with a pcb. Turn the pcb to find a small pot. With a digital multimeter (see image below) turn the pot counterclockwise until you measure 200Ohm.
The value must be between 170-210 Ohm but never 250Ohm and above.