Missing gigs

Sounds normal to me. Has to do on how you define kilobytes 1024 or 1000.
In addition there are some hidden files and folders on the disk.

In hard numbers: WD and ALL hard disk makers use the word "Gigabyte" to mean 1,000,000,000 bytes, which is right - "Giga-" as a prefix means exactly one billion. But Microsoft, like a lot of binary math people, decided long ago to define "Kilobyte" as 1,024 bytes - that is, as 2^10. As things got bigger they perpetuated the habit, so now they use the term "Gigabyte" to mean 1024 x 1024 x 1024 bytes, which is 1,073,741,824. So in their way of defining the term, 750,000,000,000 bytes is called 698.49 Gigabytes. It is EXACTLY the SAME space, but just labeled using two different measurement units, like "inches" and "pazoozas". The confusion is because they are using the same word for two different units!! But at least they are consistent. If you have a file that is 1,048,576 bytes long, M$ will call that 1.000 MB, not 1.049.

If you want to lay blame for the confusion, pick your villain. But you have NOT lost any space.