Maximize

Ok I make data disks often so that I can read from one drive while burning it to another. and it seems every data disk I have made for some time is stating that there is only 3.99 or 4gb of data when there is always 4.36 or 7.3 gb of data is there a filing system change I need to make, what do I need to do? I just wish I had an indicator when this started happening I have over 100 gb of lost data