Have an SSD drive It is one of the best options in terms of processing speed, but it has the problem that storage is very limited, and that is where Apple launches Fusion Drive.
What it does Fusion Drive is to automatically place all the apps and documents that we use the most on the SSD disk, so they will open much faster, so we will not even have to worry about placing things on one disk or another, in case we have SSD + HDD.
It works without configuration, it can be chosen in the new iMac and it is only compatible with Mountain Lion, which yes, integrates it as standard without any problem.