@Abasio: it is because for your computer 1TB = 2^40 Bytes, but for the hard drive manufacturer 1TB = 10^12 Bytes. Your computer uses binary math to compute the available space, while the harddrive manufacturer uses base 10 calculations in order to make it easier to understand because we are more accustomed to think in powers of 10
http://compreviews.a...tualHDSizes.htm