Two things affect that number, the first of which is misleading advertising. When companies list the size of a drive they are counting differently from how Windows does. In your case:
320 gb = 320,000,000,000 b
320,000,000,000 / 1024
312,500,000 / 1024
305,175 / 1024
The second thing that can affect this is if the drive comes with any pre-installed software, using up a small portion of the drive's space and possibly running an application whenever you click on the drive. Personally I don't like anything on my drive so I would recommend formatting it before use, and an excellent guide for that can be found here
Edit: That's twice today I got beat to the punch, time to take a bleeping break!
Edited by whoabuddy, 17 June 2011 - 02:02 PM.
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