why wd black better than green ?

Greens tend spin down and spin back up more often than Blacks. Blacks tend to have better logic boards with more powerful coprocessors and are almost always faster. Greens also tend to be quite a bit less reliable than Blacks. Greens are fine for mass storage of data that doesn't get accessed regularly, but don't do as well when used as an OS drive.

My answer: Because ( ͡° ͜ʖ ͡°)