"Marketing" capacity versus true capacity
It is important to note that hard drive manufacturers often use the metric definition of the prefixes "giga" and "mega." However, nearly all operating system utilities report capacities using binary definitions for the prefixes. This is largely historical, since when storage capacities started to exceed thousands of bytes, there were no standard binary prefixes (the IEC only standardized binary prefixes in 1999), so 210 (1024) bytes was called a kilobyte because 1024 is "close enough" to the metric prefix kilo, which is defined as 103 or 1000. This trend became habit and continued to be applied to the prefixes "mega," "giga," and even "tera." Obviously the discrepancy becomes much more noticeable in reported capacities in the multiple gigabyte range, and users will often notice that the volume capacity reported by their OS is significantly less than that advertised by the hard drive manufacturer. For example, a drive advertised as 200 GB can be expected to store close to 200 x 109, or 200 billion, bytes. This uses the proper SI definition of "giga," 109 and cannot be considered as incorrect. Since utilities provided by the operating system probably define a Gigabyte as 230, or 1073741824, bytes, the reported capacity of the drive will be closer to 186.26 GB (actually, GiB), a difference of well over ten gigabytes. For this very reason, many utilities that report capacity have begun to use the aforementioned IEC standard binary prefixes (e.g. KiB, MiB, GiB) since their definitions are not ambiguous.
Another side point is that many people mistakenly attribute the discrepancy in reported and advertised capacities to reserved space used for file system and partition accounting information. However, for large (several GiB) filesystems, this data rarely occupies more than several MiB, and therefore cannot possibly account for the apparent "loss" of tens of Gigabytes.
