a Gb as we all know is a measure of space (HDD or RAM)
HDD are sold unformatted, so, after formatting you will lose some space (about 7%)
GiB = an unformatted amount of space
Gb = Gigabyte, a formatted amount of space we all understand
so, HDD should really be sold as 160GiB, not 160Gb because a 160GiB HDD is only 149.8Gb ish, this so called waste of space is actually used by the HDD for error correction & stuff