UniDisk 3.5

In September 1985, Apple released its first 3.5" drive (A2M2053) for the Apple II series utilizing Sony's new 800K drive mechanism which would not be released for the Macintosh until 4 months later. The Apple UniDisk 3.5 drive contained additional circuitry making it an "intelligent" or "smart" drive; this made it incompatible with the Macintosh, despite having the identical drive mechanism that was to be used in the Macintosh. However, with the internal circuit board bypassed or removed (which consisted of its own CPU, RAM and firmware) it will operate as a standard Macintosh 800K drive.