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