Each year rains and surface erosion bring new pieces to the surface.
Slides of the mounds in Harappa and other archaeological sites in the Indus Valley by Omar Khan
This area in Mound AB, excavated in the 1920's revealed large houses thought to have belonged to upper class members of ancient Indus society.
A good counter example to "Great Granary" having been used to store grain is this actual granary popular in the villages surrounding Harappa. Grain is stored in earthen structures, and accessed as needed through a re-sealable hole at the bottom.
New possibilities might be forthcoming in the next two years [as written in 1998]. Excavations are due to begin in the small parts of the granary not completely excavated by the first archaeologists at Harappa, R. B. D. R. Sahni and M.S.
There are no answers to the purpose of this structure; current speculation suggests it may have been a palace for a ruler or a ruling group, or perhaps even a building for priests such as the later Buddhist monasteries.
Individual rooms are 15 by 6 meters long, and have sleeper walls for airspace between them. At each end of the rooms are three raised platforms.
No grain, storage containers or clay sealings such as would have been attached to goods for shipment were found in the so-called "Granaries" of Harappa or Mohenjo-daro.