b/c the wd 30 gb drive isn't really 30 gbs. It's really 30,000,000,000 bytes which isn't really 30gbs, because 1mb is 1024k.
My recently purchased wd800bb says 80gb, but it's showing 74.5. My wd600bb shows 55.7. you get about 93% of the actual space b/c Hard drive makers market these things a little deceptively. 
 If you do a search it's been explained multiple times.