What Is The iMac Fusion Drive?
The Fusion Drive combines a 128GB flash storage (SSD) with a 1TB or 3TB hard disk drive (HDD). The flash storage component of the drive stores your operating system and applications, while the HDD stores everything else. You will notice a considerable speed improvement by placing your disk-intensive tasks (like starting up your Mac or launching apps) on the flash storage section of the fusion drive. The best part is that your Mac automatically manages the fusion drive allocation, so you don't need to guess or decide which files should be stored on which partition.