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Why a kilobyte is 1000 and not 1024 bytes
(zeta.one)
This is a most excellent place for technology news and articles.
The "full available address space" goes down as the drive gets older and bad sectors are removed.
With a good drive, it might take ten or more years before you actually see the "size" of the drive shrink, but that's mostly because you 500GB drive actually had something like 650GB of storage when it was brand new.