guavasauce :
by standard, the hard drive does have 250gb of space, but due to formating and actual space usage, the drive become 235 usable gb.
Not true.
The difference in space comes between the difference between a gigabyte as interpreted by the marketing departments, and a gigabyte as interpreted by your computer. By convention, and to make their products look more attractive, drives are sold with capacities listed using decimal SI prefixes where a gigabyte (GB) is a billion bytes.
However, your computer is a binary system, and defines data sizes differently. A kilobyte, to your computer, is 2^10 bytes, or 1024 bytes. Similarly, a megabyte is a kilo-kilobyte, or 2^20, and a gigabyte is 2^30, terabyte 2^40, and so on and so forth.
This results in a compounding error that gets larger as you move through the list of prefixes. By the time you hit terabytes, you're down to about 90% of the space you were expecting.