I 'm poised to buy a new 27" (my old one is 6 years old). In the Apple store today the rep said the Fusion drives only have 24 SSD on them and the rest it old style platter -- about.. 40 to 1 ratio!. Yet online Apple says it this type of magic drive knows what you have been using and opens it up with SSD. I was leaning toward just getting a 500GB SSD which cost more, but want to ask how fusion works first, he could not explain that well. Also, he kept saying the OS is only 4GB. But on my macbook air (60GB Mac HD) it keeps running out of space (will be replacing soon), and all I have on that is 10GB of apps and 5 GB of imap mail. That's it! The rest is Library, system, and other hidden obtuse system things like Private folders which I have no idea what is in there. So to me there the OS is taking 40GB for all intents and purposes. Finally, the consensus seems to be 16 GB Ram is enough? And don't buy Apples, save $ and go crucial? Thanks. sorry title should be go fusion or all ssd, but no way to edit the title only text..?