a "120"GB drive from a maker is 120,000,000,000 Bytes (to them gigabyte is 1 billion bytes)
In reality, a KB = 1024 Bytes. 1MB = 1024 KB. 1GB = 1024 MB.
1GB in reality, is 1,073,741,824 Bytes (aka 2 to the 30th power)
2^30
so in reality, for each "GB" the Hard drive makers say they are giving you you really only get about 93.13% of 1 GB (windows calculator says 93.1322574615478515625)
This means that a 120GB Hard drive only has about 111.76GB (windows calculator says 111.758708953857421875GB)
People tend to think in Decimal ... it makes sense to see 1GB written out on paper as 1,000,000,000 Bytes. however, in binary ... that would be a very odd figure.
2^10Bytes (1024 in Decimal) is 1000000000 in Binary
2^20Bytes(1048576 In Decimas) is 10000000000000000000 in Binary ... and you can see where this is going ....