Dhyana is awareness, and Samadhi is when the awareness has become so deep, so profound, so total that it is like a fire and it consumes the whole mind and all its functionings. It consumes thoughts, desires, ambitions, hopes, dreams. It consumes the whole stuff the mind is full of.
Samadhi is the state when awareness is there, but there is nothing to be aware inside you; the witness is there, but there is nothing to be witnessed.