Disk II

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The Disk II Floppy Disk Subsystem, often rendered as Disk ][, is a 5 +1⁄4-inch floppy disk drive designed by Steve Wozniak at the recommendation of Mike Markkula, and manufactured by Apple Computer, Inc. It went on sale in June 1978 at a retail price of US$495 for pre-order; it was later sold for $595 (equivalent to $2,780 in 2023) including the controller card (which can control up to two drives) and cable. The Disk II was designed specifically for use with the Apple II personal computer family to replace the slower cassette tape storage.

Apple produced at least six variants of the basic 5 +1⁄4-inch Disk II concept over the course of the Apple II series' lifetime: The Disk II, the Disk III, the DuoDisk, the Disk IIc, the UniDisk 5.25" and the Apple 5.25 Drive. While all of these drives look different, and use four different connector types, they're all electronically extremely similar. They can all use the same low-level disk format, and are all interchangeable with the use of simple adapters, consisting of no more than two plugs and wires between them. Most DuoDisk drives, the Disk IIc, the UniDisk 5.25" and the AppleDisk 5.25" even use the same 19-pin D-Sub connector, so they are directly interchangeable. The only 5 +1⁄4" drive Apple sold aside from the Disk II family was a 360k MFM unit made to allow Mac IIs and SEs to read PC floppy disks.

This is not the case with Apple's 3 +1⁄2-inch drives, which use several different disk formats and several different interfaces, electronically quite dissimilar even in models using the same connector; they are not generally interchangeable.