confused on free space on hard drives server 2008

windows server 2008.
there are 3 partitions
c drive raid 1
d drive raid 1 + 0
g drive single hard drive

c drive and g drive are showing freespace available that doesnt sit right with me.
i have show hidden files checked from folder options.

c drive is 280 gigabytes total size. it has free space of 88.4 gigabytes.
when i calculate the folder and file size on their by hand. its only 90 gigabytes.
its like there is 100 gigabytes being used that i can't find

drive g is same problem

drive g is 931 gigabytes total with 266 gig free space
total size of files and folders is 200 gigs
where is the other 500 gigabytes

this server is and exchange server.
all exchange files are on the d drive which is the only one that seems to be showing correct size