I have Mountain Lion 10.8.5 on both my iMac and Macbook Pro. I am afraid to download either Mavericks or Yosemite. Is there a way to test Yosemite out first on my computer without actually downloading it. I heard it can be done by first downloading to HDD. How would that be done and then test it on my computer. Do I need a clean HDD? Can you tell me step by step please.