Hmm. I just checked and subtracted 8.5 GB (2^30) from 10 gb (10^9) and I get 813 MB, so that's not the problem. A lot of misunderstanding come from different products using different meanings of "gigabyte."
Can you examine the disk with, say, (under Control Panel - Administrative Tools - Computer Management) and post a picture, or a text description of the line for that volume letter, what is shown on the left in the picture line for the disk, and what is show on the right in the picture line for that partition?
Also, do a listing of the drive that will show hidden and system data. It might be your recycle bin or other system area.
Let us know what you find.