Marketing, false advertising, GB as used by computer geek vs GB as used by rest of the
world, and hidden reserve to make the drive run faster and have a safety buffer in case of
failure. Also, lost track of what sub I was in and made this not very ELI5. Sorry.
Internal computer memory (RAM or Random Access Memory) is built and accessed in
multiples and powers of two (binary). You still buy computer memory sticks in capacities
like 512 MB or 2 GB. These aren’t really “real” mega- and giga-bytes because in the old
days computer people borrowed some metric terms like kilo that meant exactly 1000 and
used them as a shorthand to refer to 1024. Even today, some operating systems and
programs (and programmers) still use the “wrong” 1024 value when calculating both
computer memory and storage.
When computer hard disk drives were invented, they weren’t electronic structures built on
powers of two, so the marketing people used the “correct” multiples of 1000 when talking
about storage capacity (decimal). If nothing else, it makes the capacity look slightly bigger,
and marketing always likes bigger. Marketing also really likes round numbers. So hard
disk drives are advertised in terms like 250 GB or 2 TB.
Solid state drives are internally more structured like computer memory (built in binary)
than mechanical hard disks (measured in decimal for convenience). A 240 GB SSD
probably has a “raw” storage capacity more like 256x1024x1024x1024 bytes (and 8x that