Return to Question

  for rows in range (0,4):
      for columns in range (0,5):
          arcpy.AddMessage('> Attempt to extract raster [row]-[col]: [' + str(rows)+ ']-['+str(columns)+']...')
          try:
              small_raster_lower_left  = arcpy.Point(large_raster_extent.XMin + (columns * window_size * large_raster_cellsize_X),
                                                     large_raster_extent.YMin + (rows    * window_size * large_raster_cellsize_Y)
                                                     )


              arcpy.AddMessage('> Raster to Numpy...')
              numpy_array  = arcpy.RasterToNumPyArray(large_raster,
                                            small_raster_lower_left,
                                            window_size,
                                            window_size,
                                            nodata_to_value=-99999
                                            )

              arcpy.AddMessage('> small raster coordinates. X: {0}, Y: {1}'.format(small_raster_lower_left.X,small_raster_lower_left.Y))

              arcpy.AddMessage('> Numpy to raster...')
              small_raster = arcpy.NumPyArrayToRaster(numpy_array,            
                                      small_raster_lower_left,
                                      large_raster_cellsize_X,
                                      large_raster_cellsize_Y,
                                      value_to_nodata = -99999
                                      )

              arcpy.DefineProjection_management(small_raster, large_raster_spatial_ref)
              small_raster_name = "small_dem_" + str(columns+rows*10)
              arcpy.AddMessage('> Success! Saving raster: ' + small_raster_name + '...')  
              small_raster.save(small_raster_name)

         except:
              arcpy.AddMessage('> Fault! Trying next window...')

  for rows in range (0,4):
      for columns in range (0,5):
          arcpy.AddMessage('> Attempt to extract raster [row]-[col]: [' + str(rows)+ ']-['+str(columns)+']...')
          try:
              small_raster_lower_left  = arcpy.Point(large_raster_extent.XMin + (columns * window_size * large_raster_cellsize_X),
                                                     large_raster_extent.YMin + (rows    * window_size * large_raster_cellsize_Y)
                                                     )


              arcpy.AddMessage('> Raster to Numpy...')
              numpy_array  = arcpy.RasterToNumPyArray(large_raster,
                                            small_raster_lower_left,
                                            window_size,
                                            window_size,
                                            nodata_to_value=-99999
                                            )

              arcpy.AddMessage('> small raster coordinates. X: {0}, Y: {1}'.format(small_raster_lower_left.X,small_raster_lower_left.Y))

              arcpy.AddMessage('> Numpy to raster...')
              small_raster = arcpy.NumPyArrayToRaster(numpy_array,            
                                      small_raster_lower_left,
                                      large_raster_cellsize_X,
                                      large_raster_cellsize_Y,
                                      value_to_nodata = -99999
                                      )

              arcpy.DefineProjection_management(small_raster, large_raster_spatial_ref)
              small_raster_name = "small_dem_" + str(columns+rows*10)
              arcpy.AddMessage('> Success! Saving raster: ' + small_raster_name + '...')  
              small_raster.save(small_raster_name)
              del numpy_array
              del small_raster
            

         except:
              arcpy.AddMessage('> Fault! Trying next window...')
              del numpy_array
              del small_raster

pd: When using DefineProjection in an empty raster (or more precisely: in a raster with all its cells null), throws an error. That's why I used try/except within the nested loops.

ps: When using DefineProjection in an empty raster (or more precisely: in a raster with all its cells null), throws an error. That's why I used try/except within the nested loops.