A subdural hematoma is a collection of blood around the brain.  When it has been there for a moderately long time, membranes may develop.  Often a neurosurgeon will drain a chronic  (over 3 weeks in age) subdural hematoma by making  small hole in the skull, and passing a drain around the brain.  Where there are membranes present,  the collection of fluid may be separated into multiple pockets, much like an ice cube tray filled with water has multiple compartments.  If so, passing a drain into one of these pockets may only drain a small amount of fluid, and a large craniotomy (opening of the skull) may be needed to remove the membranes.  Seen here is a craniotomy, with the dura incised and being reflected to the left.  The glistening surdural hematoma membranes are seen below.