On its October 23 event, Apple introduced another feature for their new iMac and Mac Mini called “Fusion Drive.” Fusion Drive is actually a hybrid drive which integrates SSD with conventional hard-disk. But Apple brought extra boosting technology with its own software to optimize the performance of the storage devices.
This Fusion Drive combines together a separate 128GB SSD and a HDD which can be either 1TB or 3TB of standard hard drive. The storage management is the main feature here for which fusion technology has been introduced which offers performance very close to that of a SSD, at least better than conventional hard-disks.
The OS and apps will be pre-installed in the flash drive by default. Users can store their documents, media files and other miscellaneous files in the HDD. Fusion drive can understand your apps priority which you use most & can move those to SSD automatically to increase the performance. You can also manually move the apps & files between the two drives. The size [3.5-inch] of the Fusion drive is more like physical hard-disks we use generally for desktops.
Courtesy: CNET And Engadget