There are at least three types of reasons for what appears to be "missing space" on a storage medium:
1. Lack of "48-bit LBA Support" - this can affect older systems (pre-2000) that can only handle smaller hard drives. The limit it imposes is that it cannot create and use any partition larger than 137 GB (or, as M$ measures it - below - 128 GB). This is NOT a problem for any SATA system, and not normally a problem with current hardware. Even where it is a limit there are ways to work around it and use virtually all the disk space available.
2. Different definitions of the same word, "Gigabyte" - this is THE MOST common problem. Hard drive manufacturers define Gigabyte to mean 1,000,000,000 bytes. But Microsoft Windows defines it as 1024 x 1024 x 1024 bytes, which is 1,073,741,824 bytes. See the 7.37% difference? So a 500 GB HDD will be recognized and labeled by Windows as having 465.66 GB. It is the SAME space and, most unfortunately, two labels using the same term, "GB" come up with different numbers! When you get into Terabytes, you get the same problem. Although the HDD makers would say a TB is 1,000,000,000,000 bytes, Windows would say no, a TB is 1,099,511,627,776, and now the disagreement over the term's definition is 9.95%! Windows would say a "1.50 TB" unit has 1.364 TB. It MIGHT even call it 1,397 GB, just to be more confusing! But its still all the SAME space - none is missing!
3. File System Overhead - on any drive, once its space is allocated to a Partition (or more than one), each must be Formatted. This installs on that Partition a set of hidden files that track the use of its space for your files. This overhead is relatively small on a large drive, but it does "use" a little bit of the total capacity.