Here come the Mebibytes
It’s the MiBs, here come the MiBs
Here come the Mebibytes 
(Mebibytes)
That you won’t remember

iirc some early hdd did use binary units for specifications (a kilobyte was 1024 bytes). marketing and greed got manufacturers to switch to the ‘larger sounding’ decimal-based units but they were called the same thing (where a kilobyte was 1000 bytes). bigger numbers sell better or some bullshit.

there’s been lawsuits over it, which is why the retail boxes and manufacturer web sites have the disclaimer about them using decimal-based units. other lawsuits resulted in the verbiage about ‘formatted capacity’ or ‘usable space’ being less probably also being present.

the distinctly different binary units themselves (such as ‘mebibyte’) were introduced and standardized by IEC ~ 1998.

Marketing says it has 1,000,000,000,000 bytes, aka 1 Trillion bytes. SI unit naming conventions dictate that is 1 Tera-of-bytes.

Computers work with powers of 2 (Like 2, 4, 8, 16, 32, …), and wouldn’t you know that 2^10 is 1024. So computer scientist will use 1024 as a stand in for Kilobyte because it makes a lot of the math elsewhere much easier.

So, when you buy a 1TB hard drive, the manufacturer is advertising it at 1 Trillion bytes. However, your operating system, like Windows or Linux or MacOS, divides by 1024 when doing the conversion like so: 1 Trillion / 1024 / 1024 / 1024. That works out to just over 931.

To help avoid confusion, the proper term for the 1024 number is a “Kibibyte,” abbreviated KiB, instead of a “kilobyte,” abbreviated KB. Similar names exist for Mebibytes, Gibibytes, Tibibytes and beyond!

TL;DR Your box is measured in TB and your computer measures in TiB because 1st number is bigger and 2nd number is actually useful.

It’s not 931 GB, it’s 931 GiB. If there’s a choice between brand A’s 931 GiB drive and brand B’s 1 TB drive, which do you think will fly off the shelves faster?

It has to do with SI units, marketing, computer science, and a few other intersections. Before marketing took over the term, kilo-, mega-, giga-, and all other methods of measuring data were powers of 2, because binary is a base 2 number system. So a kilobyte is 2¹⁰ bytes, megabytes are 2²⁰ bytes, gigabytes are 2³⁰ bytes, etc. These units are denoted as KB, MB, GB, etc.

Well, to marketing ghouls, this standard is just simply too inconvenient because it’s not in base 10. So they simply just rounded up. In small amounts of data, rounding to 1 kilobyte from 1000 bytes isn’t all that far off. But as you can imagine, this skews the correct amount of data as it starts growing to larger units.

Manufacturers were unwilling to listen to the people that understand computers, so IEEE created a new standard unit, the -ibibyte, to clear the confusion that capitalists needlessly invented. So now the units that are in powers of 2 are called kibibytes, mebibytes, gibibytes, etc. these units are denoted with KiB, MiB, GiB, etc.

The reason 1TB is equivalent to 931GiB is because the metric terabyte is 10¹¹ while the SI tibibyte is 2⁴⁰