A combination solid state drive (SSD) and magnetic disk drive. Such devices may store only the operating system in the faster solid state drive (SSD) portion, or they may automatically migrate frequently used programs and data into the SSD. These are essentially interim devices, because SSDs are still costly compared to the very mature hard drive technology. As time moves on, there will be more and more SSD-only computers and fewer ones that rely on spinning magnetic disk platters making thousands of rotations per minute. See hybrid drive, Fusion Drive, solid state drive, magnetic disk and flash memory.