You need to burn it and/or pulverize it to the point it's turned to ashes. The only way to be certain.
Discussion
A multiple step process for sure. Microwave is good for simple sd cards and flash memory when time is of the essence. Fire is great, but kinda toxic, still, it works. Hammer, sandpaper, then drill for mechanical drives etc, grind to a pulp or thermite. So many fun ways to destroy data :)
