Apple Hard Disk 20SC
| Apple Hard Disk 20SC | |
|---|---|
 |
|
| Apple Hard Disk 20SC (All About Apple museum) | |
| Manufacturer | Apple Computer, Inc. |
| Type | External SCSI hard disk drive |
| Release date | September 1986[1] |
| Price | US$1,299 (1986)[1] |
| Interface | SCSI, two 50-pin connectors |
| Compatible | Macintosh Plus, Macintosh SE, Macintosh II; Apple IIe and Apple IIGS with the Apple II SCSI card |
| Dimensions | 246 × 78 × 266 mm (9.7 × 3.1 × 10.5 in) |
| Weight | 3.63 kg (8 lb) maximum |
| Predecessor | Apple Hard Disk 20 |
| Power | 85-270 V AC, 47-64 Hz, 60 W |
| Storage | 20 MB formatted |
The Apple Hard Disk 20SC is Apple's 20 MB external SCSI hard disk. It connects to the SCSI port of the Macintosh Plus, SE and II, and to an Apple IIe or IIGS through the Apple II SCSI card.[2] Apple introduced it in September 1986 at US$1,299.[1] It uses SCSI, where the earlier Apple Hard Disk 20 plugs into the floppy port. Apple sold it as part of the Apple Hard Disk SC range; the UK packing list for the range covers the Hard Disk 20SC, 40SC, 80SC and 80SC with A/UX, all shipped with the Hard Disk 20SC Owner's Guide, a SCSI system cable and a SCSI terminator.[3]
The 20SC, 40SC, 80SC and 160SC share one case, power supply and fan, and differ in the drive mechanism inside. Apple's service procedures cover all four together.[4] Their care, take-apart and repair are on Apple Hard Disk SC General Maintenance, Apple Hard Disk SC Troubleshooting Guide and Apple Hard Disk SC Capacitor Replacement Guide.
Specifications
[edit | edit source]| Item | Value |
|---|---|
| Formatted capacity | 20 MB[2] |
| Data surfaces | 4, one head per surface[2] |
| Blocks | 39,360[2][5] |
| Block size | 512 bytes in Apple's specification article; 532 bytes per block in the owner's guide[2][5] |
| Sectors per track | 32[2] |
| Average access time | 65-85 ms[2] |
| Head positioning | Open-loop stepper actuator[6] |
| Data transfer rate | Up to 1.25 MB/s[2] |
| Rotational speed | 2,744 rpm[2] |
| Start-up and spin-down | 9 s and 20 s[2] |
| Line voltage | 85-270 V AC RMS, 47-64 Hz, adapting automatically[2] |
| Power consumption | 60 W[2] |
| Operating temperature | 10-40 °C (50-104 °F)[2] |
| Relative humidity | 20-80 %, non-condensing[2] |
| Height × width × depth | 78 × 246 × 266 mm (3.1 × 9.7 × 10.5 in)[2] |
| Maximum weight | 3.63 kg (8 lb)[2] |
Apple measured real SCSI transfer rates of about 265 KB/s on a Macintosh Plus, 660 KB/s on an SE and 937 KB/s on a Macintosh II for its HD SC drives, against the 1.25 MB/s maximum. The interleave factors Apple used were 3:1 on the Apple II and Macintosh Plus, 2:1 on the SE and 1:1 on the Macintosh II.[6]
Mechanisms and revisions
[edit | edit source]Apple used both 3.5-inch and 5.25-inch 20 MB mechanisms.[4] The 5.25-inch drive in a 1988 unit restored by Adam Wilson was a Seagate ST-225N.[7]
The 20 MB drive came in two revisions, A and B, which look the same to the owner but must be replaced like-for-like. On a drive in an internal frame, a Revision A board is mounted component side up and a Revision B board solder side up.[4] Apple's 1991 parts list gives the external 20 MB hard disk assembly as 661-0342 (Revision A) and 661-0612 (Revision B).[4] Apple's later Hard Drives Service Source gives 661-0373 for the 3.5-inch Revision A drive and the same 661-0612 for Revision B.[8]
One version of the 3.5-inch 20 MB drive needs a new controller EPROM to work in a Macintosh II or IIx. Apple's check is for a 28-pin EPROM at U12 labelled "2.6A"; if it carries that label, it is replaced with the upgrade EPROM, notch towards the 50-pin SCSI connector.[4]
Using it with an Apple II
[edit | edit source]The Apple II needs an Apple II SCSI card.[2] Because ProDOS handles volumes of up to 32 MB, the 20SC is the only Apple SCSI drive that an Apple IIe can use without partitioning.[4] An unenhanced Apple IIe can read the drive after starting from another disk but cannot start up from it; Apple's fix was the Apple IIe Enhancement Kit (A2M2052).[9] To format it from an Apple II, Apple recommends System Disk 3.0 on the IIGS and on a platinum IIe with a Rev C or Rev E board, and System Utilities 2.1.1 on an unenhanced IIe.[10] By 1991 Apple's card for the purpose was the Apple II High-Speed SCSI Card, A0220LL/A.[11]
Setting up
[edit | edit source]Keep the vents at the sides and back of the drive clear. Beside a Macintosh, put the drive on the right: on the left it risks magnetic interference from the Macintosh's power supply and video circuitry. Do not stand a monitor on it.[5]
Set the priority number (SCSI ID) on the switch at the back with an opened paper clip. Every SCSI device on the bus needs a different number.[4] Switch the drive on before the computer; it is ready within 20 seconds.[4] With two or more 20SCs on a Macintosh, switch them all on and leave them for at least 10 seconds before switching on the Macintosh.[5]
Apple's own manuals disagreed on where the terminator goes. The 20SC owner's guide shows it on the drive's spare SCSI port, while the Apple SCSI Cable System manual puts it between the cable and the drive. Apple's support article says both work but recommends the cable-system manual's arrangement, a terminator on the SCSI input of the first device and, where there is more than one device or more than 10 feet of cable, a second terminator on the last device.[12]
Known faults
[edit | edit source]- Drive not seen, or sluggish, after a mechanism has been replaced: Apple's replacement modules ship with terminating resistor packs for internal use, which must come out for an external case.[4]
- The LED stays on or keeps flashing after start-up: Apple says the hard disk assembly is probably bad. The LED is off when the drive is ready.[4]
- Switching off, or a power failure, while the light is blinking can damage the disk's contents, which then needs reinitialising and restoring from a backup.[5]
- Ageing power supply electrolytics and overheated resistors in the power supply: see Apple Hard Disk SC Capacitor Replacement Guide.[7]
Full diagnosis is on Apple Hard Disk SC Troubleshooting Guide.
Documentation
[edit | edit source]- Apple Hard Disk 20SC Owner's Guide: Apple's 1986 owner's guide (030-3138-A), with set-up for Macintosh and Apple II, owner troubleshooting and specifications.[5]
- Apple Service Technical Procedures Cross Family Peripherals Volume One: Apple's 1991 service procedures for the SCSI hard disk drives, with take-apart, diagnostics, troubleshooting flowcharts, additional procedures and the illustrated parts list.[4]
- Apple External Hard Drive SC Service Source: Apple's 1995 take-apart volume for the 20SC, 40SC, 80SC and 160SC case.
- Apple Hard Drives Service Source: general procedures for Apple drive mechanisms, including the Revision A and B identification.[8]
Gallery
[edit | edit source]-
Apple's exploded view of the case shared by the 20SC, 40SC, 80SC and 160SC
.png)
Related pages
[edit | edit source]- Apple Hard Disk SC General Maintenance
- Apple Hard Disk SC Troubleshooting Guide
- Apple Hard Disk SC Capacitor Replacement Guide
- Apple Hard Disk 40SC
- Apple Hard Disk 80SC
- Apple Hard Disk 160SC
- Apple Hard Disk 20
- Hard Drive Maintenance and Repair
References
[edit | edit source]- ↑ 1.0 1.1 1.2 Wikipedia, "Hard Disk 20SC", retrieved 2026-10-04. Source for the September 1986 introduction and the US$1,299 price, citing U-M Computing News (27 October 1986) and InfoWorld.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 Apple Computer, "Apple Hard Disk 20SC: Specifications (Discontinued)", Tech Info Library article 1931, created 1 October 1986, reviewed 10 September 1987, revised 13 September 1993, in the archive.org collection Apple's Technical Information Library Articles, https://archive.org/details/APPLE_TIL_ARTICLES, file TIL00001_to_TIL60496.zip. Source for every figure in the specification table except where another source is given, the two 50-pin connectors and pass-through expansion port, and the list of compatible computers.
- ↑ Apple Computer, Apple Hard Disk SC Packing List, B030-3139-C, archive.org item apple-hard-disk-sc-packing-list-b030-3139-c (retrieved 2026-10-04). Lists the 20SC, 40SC, 80SC and 80SC with A/UX, the Hard Disk SC Setup disk, the Hard Disk 20SC Owner's Guide, the SCSI Cable System manual, power cord, SCSI system cable and SCSI terminator.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 Apple Computer, Apple Service Technical Procedures, Cross Family Peripherals, Volume One (PN 072-0230, 1991), SCSI Hard Disk Drives chapter, hosted as File:Apple Service Technical Procedures Cross Family Peripherals Volume One Apr 1991.pdf and described at Apple Service Technical Procedures Cross Family Peripherals Volume One. Basics, pp. 1.3-1.6 (PDF pp. 178-181): 3.5-inch and 5.25-inch drive matrix, the 20SC as the only drive usable on an Apple IIe without partitioning, the priority switch, switching on within 20 seconds; Take-Apart, pp. 2.2-2.10 (PDF pp. 188-196): one case for all external drives, Revision A and B; Troubleshooting, p. 4.3 (PDF p. 238): LED and resistor packs; Additional Procedures, pp. 6.2-6.3 (PDF pp. 247-248): 20SC EPROM upgrade; Illustrated Parts List, IPL.3 and IPL.5 (PDF pp. 256, 258).
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Apple Computer, Apple Hard Disk 20SC Owner's Guide (030-3138-A, 1986), hosted as File:Apple Hard Disk 20SC Owners Guide 1986.pdf and described at Apple Hard Disk 20SC Owner's Guide. Printed p. 4 (PDF p. 14): placement and ventilation; pp. 22-25 (PDF pp. 32-35): operating environment, cleaning, the high-voltage warning, problem descriptions and the Installer's drive test; p. 27 (PDF p. 37): specifications, including 532 bytes per block and 39,360 blocks.
- ↑ 6.0 6.1 Apple Computer, "Apple HD SC: Specifications (Discontinued)", Tech Info Library article 2791, created 28 April 1988, revised 13 September 1993, APPLE_TIL_ARTICLES collection (archive.org), file TIL00001_to_TIL60496.zip. Source for the open-loop stepper actuator and 65 ms access of the HD-20 SC, the rotary voice-coil actuators of the HD-40 SC and HD-80 SC, the interleave factors and the measured transfer rates.
- ↑ 7.0 7.1 Adam Wilson, "1988 Apple 20SC External HDD Repair & Restoration", Adam's Vintage Computer Restorations, 25 January 2023, retrieved 2026-10-04. Source for the Seagate ST-225N mechanism, the internal 5 V/12 V switch-mode supply and 12 V fan, and the power supply capacitor list.
- ↑ 8.0 8.1 Apple Computer, Hard Drives (Service Source), 1995, Upgrades, "Identifying 20SC Rev A & B", pp. 6-7. Hosted as File:Macintosh Disk Drive and UniDisk 3.5.pdf and described at Apple Hard Drives Service Source.
- ↑ Apple Computer, "Hard Disk 20SC: Can't Be Startup Device for Unenhanced Apple IIe", Tech Info Library article 7582, created 19 April 1991, APPLE_TIL_ARTICLES collection (archive.org), file TIL00001_to_TIL60496.zip.
- ↑ Apple Computer, "HD-20SC: What You'll Need to Format It With an Apple II", Tech Info Library article 2199, reviewed 19 January 1988, APPLE_TIL_ARTICLES collection (archive.org), file TIL00001_to_TIL60496.zip.
- ↑ Apple Computer, "Hard Disk 20SC: How to Connect to an Apple IIe/IIGS", Tech Info Library article 8512, created 25 June 1991, APPLE_TIL_ARTICLES collection (archive.org), file TIL00001_to_TIL60496.zip.
- ↑ Apple Computer, "Hard Disk 20SC: Connecting a terminator", Tech Info Library article 662, reviewed 30 September 1987, revised 29 September 1990, APPLE_TIL_ARTICLES collection (archive.org), file TIL00001_to_TIL60496.zip.