I want to buy a macbook pro 13" used, but want to wipe everything off the hard drive and update to the newest OS (Yosemite) if possible. Potentially even install a better hard drive. I did some poking around but was wondering how if I take out the old drive and install a new one, that I can reinstall the OS without a CD? I have an external drive, but not sure on that process. Would likely want to swap out to a SSD but it wouldn't contain the OS on it. Wouldn't have TimeMachine setup either. Any tutorials on this? Thanks in advance.