Hello, I want to change my macbook pro to ssd. I just use it for internet so I don't need large storage. I have been told the macbook pro 13 has two hard drives. One for os and one for files, i have not opened mine yet. Is this true? I just want speed and can only get one ssd. What is the smallest size ssd i can put in the macbook pro 13 that will work? Thank you.