Apple's Fusion Drive is often the source of confusion amongst computer users. This is especially true for novice users as well as those who have recently switched to the Mac platform. However, as you'll find out, the Fusion Drive is really nothing more than a traditional hybrid drive that you're probably already familiar with.
What is the Apple Fusion Drive?
Originally announced in October 2012 and first featured in iMacs and Mac Minis running OS X Mountain Lion, the Apple Fusion Drive attempts to bring Mac users the best of both storage mediums through a combination hard disk drive (HDD) and flash-based solid-state drive (SSD).
From a user's perspective, the separate drive architectures are seamless. Since the OS automatically manages file allocation by storing frequently accessed files on the flash-based portion and those that are infrequently accessed on the HDD section. Since SSD technology features faster access times as well as increased read and write speeds when compared to traditional HDDs, these files can be opened, edited, and saved at faster speeds than the traditional HDD format.
The Apple Fusion Drive also responds to user access patterns and modifies its storage based on actual usage. If a file that is stored on the SSD portion of the drive suddenly becomes unnecessary or infrequently accessed, it is automatically migrated to the slower, HDD portion. Conversely, if a particular file suddenly requires frequent access, the system automatically moves it to the SSD portion.
Although the Apple Fusion Drive was viewed as a major upgrade to Mac computers, and it was even featured in subsequent models of the iMac and Mac Mini, developers never implemented the technology in other Apple products. Instead, they opted for hard drives that are entirely flash-based in some of their more recent releases.
The amount of HDD-based and flash-based storage also varies depending on the year and model of device. For example, iMacs and Mac Minis manufactured in mid-2015 or earlier feature 128 GB of flash-based storage and either 1 TB or 3TB of traditional, HDD-based storage. Later models tend to vary, with flash-based storage ranging from 24 GB to 128 GB and HDD-based storage ranging from 1 TB to 3 TB.
In order to keep costs at a minimum, most of Apple's Fusion Drives feature a relatively small amount of SSD capacity. Since this technology is still relatively new, especially when compared to the current generation of HDD technology, consumer-level SSDs just aren't able to match the capacity of their HDD counterparts.
As such, Apple's Fusion Drives lets uses take advantage of SSD-based storage for their important files - including critical operating system files - and HDD-based storage for long-term archival and general data storage. It effectively gives Apple users a way to benefit from the speed and efficiency of flash-based storage while still maintaining enough capacity - via the HDD - to store plenty of personal files.
While SSDs are becoming more affordable and available in larger capacities, most aren't enough to handle an entire Mac OS as well as your application files, games, digital pictures, music, and videos. Although they are expected to completely replace HDD technology in the future, we're currently at the point where a hybrid or Apple Fusion Drive is one of the best available options.
You may read more about Fusion Drive in Wikipedia: Fusion Drive.