Editing Disk II

{{Infobox computer peripheral
| image = [[File:Disk II.jpg]]
| caption = Disk II Floppy Disk Subsystem (Model A2M0003)
| manufacturer = Apple Computer, Inc. (Shugart/Alps mechanisms)
| type = Floppy disk drive
| released = June 1978
| discontinued = 1985
| price = US$595 (with controller)
| interface = 20-pin ribbon to controller card
| compatible = Apple II, Apple II Plus, Apple IIe
| dimensions = ~215 mm × 180 mm × 90 mm
| weight = ~2 kg
| predecessor = Cassette tape storage
| successor = [[Apple DuoDisk]]
| model = A2M0003
}}

The '''Disk II Floppy Disk Subsystem''' (often rendered '''Disk ][''', model A2M0003) is a 5¼-inch floppy disk drive designed by [[Steve Wozniak]] for the [[Apple II]]. Released in June 1978 at US$595 (including controller card), the Disk II was the cheapest floppy disk system sold up to that point and was instrumental in establishing the Apple II as a serious business computer capable of running [[VisiCalc]].

== Background ==

The Apple II originally used cassette tape storage like other microcomputers of the era. Apple investor Mike Markkula asked Wozniak to design a disk drive after finding that a checkbook-balancing program he had written took too long to load from tape.

Wozniak studied IBM and North Star disk controller designs, then created a dramatically simplified controller using only a handful of chips—about one-tenth the chip count of existing controllers. He called the Disk II "my most incredible experience at Apple and the finest job I did."

== Design ==

=== Drive Mechanism ===
The original Disk II used modified Shugart SA-400 mechanisms:
* Apple purchased bare mechanisms without standard controller boards
* Wozniak's controller board replaced Shugart's
* Apple logo stamped on faceplate
* Early production: 30 drives assembled per day by two people
* By 1982, switched to Alps mechanisms for cost reasons

=== Controller Card ===
The Wozniak-designed controller features:
* Software-driven operation (total user control over format)
* Group Coded Recording (GCR) encoding
* 13 sectors per track (DOS 3.2) or 16 sectors per track (DOS 3.3)
* Supports two drives per card
* Up to 14 drives possible (7 cards × 2 drives)

=== Head Load Solenoid ===
The standard SA-400 had a head load solenoid to lift heads when not accessing. Wozniak removed this, proving it unnecessary for 5.25" drives. Most manufacturers subsequently omitted the feature.

== Storage Format ==

{| class="wikitable"
|-
! DOS Version !! Sectors/Track !! Capacity !! Encoding
|-
| DOS 3.2.1 and earlier || 13 || 113.75 KB || GCR "5 and 3"
|-
| DOS 3.3 || 16 || 140 KB || GCR "6 and 2"
|-
| ProDOS || 16 || 140 KB || GCR "6 and 2"
|}

The 16-sector upgrade (1980) modified only controller card firmware—no drive changes required. This format provided nearly 20% more storage than standard FM drives.

=== Flippy Disks ===
Users commonly cut a second write-enable notch to use the disk's opposite side, effectively doubling capacity. Commercial software often shipped on such "flippy" disks.

== Specifications ==

{| class="wikitable"
|-
! Parameter !! Value
|-
| Model number || A2M0003
|-
| Capacity || 113.75 KB (DOS 3.2) / 140 KB (DOS 3.3)
|-
| Media || 5.25-inch single-sided floppy
|-
| Tracks || 35
|-
| Sectors || 13 or 16 per track
|-
| Encoding || GCR (5&3 or 6&2)
|-
| Transfer rate || ~15 KB/s
|-
| Interface || 20-pin ribbon cable
|-
| Controller card || Required (slot 6 typical)
|-
| Case || Beige-painted metal
|-
| Price (1978) || US$595 with controller
|}

== Variants ==

Apple produced multiple 5.25-inch drive variants:

{| class="wikitable"
|-
! Model !! Name !! Year !! Notes
|-
| A2M0003 || Disk II || 1978 || Original, full-height
|-
| A3M0004 || Disk III || 1980 || Apple III, plastic case
|-
| A9M0108 || DuoDisk || 1984 || Dual half-height drives
|-
| A2M4050 || Disk IIc || 1984 || Half-height for IIc
|-
| A9M0104 || UniDisk 5.25 || 1985 || Single half-height
|-
| A9M0107 || Apple 5.25 Drive || 1986 || Platinum gray
|}

All variants use the same low-level disk format and are interchangeable with simple adapters.

== Connection ==

The 20-pin ribbon cable connector is easy to misalign, which shorts an IC in the drive. A damaged drive will delete data from any inserted disk as soon as it spins—even write-protected disks. Apple printed warning messages about proper connector installation.

Later drives adopted the DB-19 connector standard to prevent this issue.

== Copy Protection ==

The software-driven controller allowed elaborate copy protection schemes:
* Total control over disk format
* Non-standard sector layouts
* Timing-based protection
* Spiral tracks
* Half-tracks

This made the Apple II platform notorious for copy protection innovation.

== Bell & Howell Version ==

Apple manufactured a black-painted Disk II to match the Bell & Howell version of the Apple II Plus, sold to educational markets.

== Maintenance ==

=== Head Cleaning ===
* Use 5.25-inch head cleaning disk
* Single head on bottom of mechanism
* Clean periodically, especially with read errors

=== Speed Adjustment ===
The drive motor speed may drift over time:
* Requires oscilloscope or specialized software
* Adjustment potentiometer on mechanism
* Incorrect speed causes read/write errors

=== Common Issues ===

{| class="wikitable"
|-
! Problem !! Cause !! Solution
|-
| Won't read || Dirty head, speed drift || Clean head, adjust speed
|-
| Destroys disks || Misaligned connector damage || Replace damaged IC
|-
| Intermittent || Worn drive belt || Replace belt
|-
| No spin || Motor failure || Replace motor or mechanism
|}

== Legacy ==

The Disk II was revolutionary:
* Cheapest floppy system to date
* Enabled VisiCalc and business applications
* Established 5.25" format on microcomputers
* Wozniak's most elegant engineering achievement

The drive's success was crucial to Apple II's dominance in the late 1970s and early 1980s.

== Collecting ==

=== Identification ===
* Model A2M0003 on controller card
* Full-height beige metal case
* 20-pin ribbon cable (not DB-19)
* Shugart or Alps mechanism

=== Condition Assessment ===
* '''Mint:''' Original box, functional, clean
* '''Excellent:''' Functional, minimal wear
* '''Good:''' Functional, cosmetic wear
* '''Fair:''' Needs adjustment but works
* '''Poor:''' Non-functional

=== Controller Card ===
The controller card is essential—the drive alone is incomplete. Cards are reusable with later drives via adapters.

== See Also ==
* [[Apple II]]
* [[Apple DuoDisk]]
* [[Apple UniDisk 3.5]]
* [[Apple 5.25 Drive]]

== References ==
* Wikipedia — Disk II
* Byte Magazine (January 1985) — Wozniak interview
* folklore.org — Macintosh development history

[[Category:Apple Vintage Accessories]]

{{Navbox-AppleVintageAccessories}}