EC2 EBS usage different between two instances

I have two Windows instances with the standard 30G EBS drive. They were created from the same AMI. After some runtime, one instance shows 27G used space and the other 15G. I have manually gone through and looked at the properties of all of the directories on the drives and they are basically the same. I have manually added up the size of each directory on the full drive, and it totals about 15G - not 27.
So my questions is, I have one drive that is hiding 10G somewhere. How do I find it? I ran the disk cleanup tool, but it only found a little.