Why are harddrives always smaller than advertised?

Excellent explanation for why bytes aren't always full bytes in measurements of harddrives.  WiebeTech provides quality storage solutions [I am not a vendor but a satisfied customer] and there are a number of 'whitepapers' available on its website.  One of them answers the question of when 1 billion does not equal 1 billion.  You can find the whitepaper, and others, here.