Missing HDD Space

"My hard drive isn't as big as it should be... why?"

A few things could be going on here.. 

First, lets look at it this way.. your Windows OS is using binary terabytes and gigabytes rather than decimal, but they didn't bother to write "binary gigabyte", nor did the HDD manufacturer bother to write "decimal gigabyte". Both are correct in a sense, and you aren't losing anything, you're just using a slightly different unit system. It would be like measuring something in nautical miles vs miles.
Keep in mind that the hard drive manufacturers are using the accurate description of the terms–the prefix giga, for instance, means a power of 1000, whereas the correct term for powers of 1024 is gibibyte, though it isn’t often used. Unfortunately, Windows has always calculated hard drives as powers of 1024 while hard drive manufacturers use powers of 1000.

Secondly, when you install Windows on a HDD, A small portion of the drive is used (and not reported as such) by Windows Shadow Copy Storage and other system partitions, such as the MBR (master boot record) and Recovery Partition.

Let's check it out! The quickest way to check this is to launch the Disk Management tool is by hitting Start, typing “partition” into the search box, and then clicking the “Create and format hard disk partitions” option that comes up.

Now, there is one other issue that could cause this problem, so let's take a look...

While you're in the Disk Management Window, (following steps above) 

Please follow these steps:

1) Right Click on the (C:) Partition
2) Select Extend Volume

3) Select size increase (default is max size)

A few more clicks and then the Drive OS partition will be set to the correct size based on the HDD.

Let us know if you need any further assistance... Hope this helps!