Calculating Free Space at End of Disk

Hey,

I'm currently trying to calculate the free space at the end of a physical hard disk using AutoIT

I'm experiencing a problem not with AutoIT but with Windows Partitions and disks and was wondering if anyone had any insight on this..

I have a hard disk which WMI reports having a size of 256052966400 bytes (31130 Cylinders * 255 Heads * 63 Sectors * 512 Bytes per Sector).

This Hard Disk has 2 partitions:

Part 1:

Offset: 1048576 Bytes
Size: 107374182400 Bytes

Part 2: 

Offset: 107375230976 bytes

Size: 148682833920 Bytes

The problem I'm experiencing is that 1048576 + 107374182400 + 148682833920 = 256052966400 Total Partitioned Bytes

256052966400 Drive Size
256058064896 Partitioned Bytes

-------------------

-5098496 Free Space

Whenever I sum the size of the partitions I end up with a value that's larger than the reported disk size.

Any ideas why I might be encountering this?

Thanks,

-Easton