External HD or Usb Stick for buffering\caching

I have searched this over and over and watched many youtube videos, but yet i cannot get the answer i am looking for.

When setting <cachemembuffersize> to 0 it should buffer without issue to your external HD or stick, yet when i insert a drive to my PI3 and set my <cachemembuffersize> to 0 and i start a movie i can see no activity on my drive the light will tell me if there is any.

Should the drive be prepared for this, is there a setting somewhere.