Hello,
i dont really know the exact amount that is left out but i think its 5% for the normal retail drives and 10% of the actuall capacity for the high end drives like the Extreme pro.
you can contact sandisk for further detailed information about the case through this link
http://kb.sandisk.com/app/ask/session/L3RpbWUvMTQ1MzExMzM4MC9zaWQvejM2YTFRR20%3D
