there is a good reason why 20GB HDs only have 18.6GB actual space
the manufacturers use a metric system to calculate capacity
whereas windows will use a binary system
by 20GB they mean to say your hard drive will hold 20,000,000,000 Bytes using 1000 Bytes to a KB, 1000KB to a MB and 1000MB to a GB
a pc will read 1024 Bytes to a KB, 1024KB to a MB and 1024MB to a GB
this means a true 20GB hard drive should have 21,474,836,480 Bytes
divide 20,000,000,000 by 1024, divide your answer by 1024 again and then divide that answer by 1024 once more and see what answer you get