Drum drying and spray drying refer to making milk powder. buffalo milk id drum dried and cow milk is spray dried. The buffalo milk has more fat and hence difficult to spray dry it. So the milk is allowed to run on to a rotating drum in a hot chamber. The cow milk is usually sprayed through a narrow nozzle into a hot chamber. Both processes yield milk powder.
For drum drying you put wet stuff in the drum and blow warm air through the drum so your stuff dries.
For spray drying you put stuff in a blender the spray a thin layer of stuff onto whatever you prefer and let it dry. Works fast because the ratio of wet stuff to wet superficies is good.