The advertised drive size that appears on the box or the label is simply
that "advertising". All drives have an actual size that differs from the
advertised or "rounded up" size that the company uses to advertise with.
After all 320 GB sounds much better than 305 GB. So the drive itself is
physically going to be less than 320 GB and could be closer to what you see.
To add to this reality when a drive is partitioned and formatted a certain
amount of space will be used up by the partitioning and formatting process.
check this link
http://www.lockergnome.com/nexus/wi...e-space-on-my-hard-drive-what-was-advertised/
and read some of the posts here which likely relate very closely to your
drive size issue
http://forums.anandtech.com/messageview.aspx?catid=27&threadid=2011883&STARTPAGE=1&enterthread=y