One of the most interesting renew from Apple next to the new iMac, Mac Mini, MacBook Pro, iPad 4 and iPad Mini announced was called Fusion Drive, this is not an entirely new system that tries to be a bridge function between an SSD and a hard drive. If you are using a Fusion Drive then you will not like normal two different disks seen in OS X, but the disk is seen as a volume while it benefits from the speed of the SSD and the extra storage capacity of the hard disk .
The most important of the Fusion Drive is also the Apple-developed software that allows system files, operating system and key applications are stored on the faster SSD and other data such as music, movies and documents are saved on the hard disk.
Attached are a 120GB SSD (disk1) and a 750GB HDD (disk7) to my Mac. I attached the SSD via SATA to be sure that the system could figure out that it’s a SSD via SMART. The HDD is attached via USB. USB I chose to clearly see a difference in speed.
Fortunately, it now appears that not only new Mac owners can use the Fusion Drive for Mac developer Patrick Stein has managed to own a Fusion Drive to create through some Terminal commands. The developer configured an SSD and a USB hard drive on his system and managed to combine both in a single volume, a volume that is identical to the Fusion Drive.
Stein continued the process data to a SSD to write and to see how the data to the hard disk was transferred after the SSD was filled. Throughout the time the data in the monitor Stein saw that the data is automatically written to the SSD for faster access. When this process was stopped by the developer, the system began to write the data back to the hard disk. [via MacRumors]