oh I can't count how many times I've had to explain this to people. your OS's idea of "2 TB" is different from the manufacturer's, because the OS uses a base of 2 when calculating data, and the manufacturer uses a base of 10. for example, 2^10 bytes is also known as one kilobyte, because it comes out to about 1000 (but it's actually 1024). since the prefix is "kilo" the manufacturer is actually correct, but it's easier from an engineering aspect to deal with a base of 2 since computers are binary digital machines. so, 2^20 is what your computer considers a Mega (as in, mega bytes), 2^30 is a giga, and 2^40 is a tera. since these units are powers of ten in every other convention than computers (10^3 is kilo, 10^6 is mega, 10^9 is giga, 10^12 is tera), the manufacturers use these units in their correct sense, which winds up giving you less that what your computer considers those units to be.
tl;dr: what's a 2 TB drive go for these days anyway, $30? cry more.