Disk II

Disk II
Disk II Floppy Disk Subsystem (Model A2M0003)
Manufacturer	Apple Computer, Inc. (Shugart/Alps mechanisms)
Type	Floppy disk drive
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

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

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:

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

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