from wiki:
https://en.wikipedia.org/wiki/Hard_disk_drive
The total capacity of HDDs is given by manufacturers using
SI decimal prefixes such as
gigabytes (1 GB = 1,000,000,000 bytes) and
terabytes (1 TB = 1,000,000,000,000 bytes). This practice dates back to the early days of computing; by the 1970s, "million", "mega" and "M" were consistently used in the decimal sense for drive capacity. However, capacities of
memory are quoted using a
binary interpretation of the prefixes, i.e. using powers of 1024 instead of 1000.
Software reports hard disk drive or memory capacity in different forms using either decimal or binary prefixes. The
Microsoft Windows family of operating systems uses the binary convention when reporting storage capacity, so an HDD offered by its manufacturer as a 1 TB drive is reported by these operating systems as a 931 GB HDD.
Mac OS X 10.6 ("
Snow Leopard") uses decimal convention when reporting HDD capacity. The default behavior of the df
command-line utility on Linux is to report the HDD capacity as a number of 1024-byte units.
The difference between the decimal and binary prefix interpretation caused some consumer confusion and led to class action suits
against HDD manufacturers. The plaintiffs argued that the use of decimal prefixes effectively misled consumers while the defendants denied any wrongdoing or liability, asserting that their marketing and advertising complied in all respects with the law and that no class member sustained any damages or injuries.