JLatham (removethis) wrote:
For more real-world numbers, consider these:
I have a 250 GB (advertised size) drive that shows up as 220 GB in Windows
I have a 500 GB (advertised size) drive that shows up as 465 GB in Windows

While you are correct that sometimes a "kilobyte" is 1024 and sometimes
it is 1000 (and similarly for megabytes and gigabytes), I do not
believe that explains the difference you describe above -- at least,
not entirely. For example, if "250 GB" were 250,000,000,000, that
would be 232.8 GB, where GB is 1024*1024*1024.

I believe that some or most of the disparity that you observe is due to
the difference between "formatted" and "unformatted" numbers. The
ratio of formatted-to-unformatted space depends on the sector size,
which varies from one OS to another. So disk manufacturers usually
advertise the unformatted number.

In addition, as part of the formatting process, operating systems set
aside a certain amount of space for directory and other purposes. That
space is usually not included in the available space that the OS
reports to the user.

Once upon a time, the convention was to use "K" for 1024 and "k" for
1000. I do not think we can rely on that convention anymore. So both
of you are correct: KB, MB and GB may be powers of 1000 or 1024.
Y'never know for sure.