Apple brand HD20, HD20 SC Info

[floppy hard drive] The HD20 Apple hard drive (HD20) connects to a Mac 128K, 512K, or later Mac via the 19-pin DB-19 floppy connector. It is not SCSI.

[HD20 hard drive] The Apple Hard Disk 20 SC (HD20 SC) is a SCSI drive, which connects to the Mac Plus and later Macs via the Apple Mac 25-pin DB-25 SCSI connector. The external drive has a pair of SCSI-1 connectors, so a DB-25 to SCSI-1 cable makes the connection.

This Web page has some technical info on these drives. Prices and cosmetic condition for the HD20 and HD20 SC I offer, are all listed in my Mac collectables section.

We offer some older system software to support the HD20 on our Mac software page. The HD20 SC and other SCSI drives operate on Macs like other SCSI drives. If you want links to Apple's site for some older Mac software, and to other Mac Web sites, check my Mac links page.

Apple HD 20 (floppy connected) hard drive

SUMMARY: The Mac HD20 is NOT a SCSI-connected drive. It has a 19-pin connector that connects to the Mac *FLOPPY PORT*. It is also the only hard drive that will work with an unmodified Mac 512K. It has its own cable too. But you may need a system extension to operate the HD20. Check our operating system section of our software page for old system software for the HD20 and the 128k & 512K.

Most people who are interested in the HD20 want to use a hard drive with their Mac 128K, Mac 512K or Mac 512KE. Those Macs, if unmodified, DO NOT HAVE (normally) a SCSI drive connector (DB-25) or SCSI controller inside. The Mac SCSI connector on later systems is a 25-pin connector, the DB-25. The normal 512K or 128K Mac has only a 19-pin connector for the external floppy. So Apple designed the HD20, which uses the external FLOPPY connector (DB-19) and a special external hard drive designed to use that floppy connector; and additional software to run that drive on those Macs. That Apple external hard drive is an Apple HD20, which has an internal 20MB hard drive which is special to that product - not a standard SCSI hard drive which is internal to later Macs and to later external SCSI hard drives from Apple and other companies.

But it is possible a Mac 128K or 512K may have a product in it which HAS a dB-25 SCSI connector, and so allows the use of external SCSI drives. Or it's possible a 512K or 128K was upgraded to a Mac PLUS, which has a DB-25 SCSI connector. Examine your Mac and confirm if it has a SCSI connector and a SCSI controller inside.

I've accumulated some notes from the Web about the HD20. Check this file for some of them. For general information, here is something from Paul "neon" Gooch:

The Mac 128 and 512k will need [the HD20 INIT] extension to use the HD20.
The Mac 512ke and Plus do not. Basically the 64k ROM, 400k drive Macs need the
extension and 128k ROM, 800k drive Macs don't. The 128k ROM as well as the
HD20 INIT give the required HFS instead of MFS of the 64k ROM. In fact your
Mac512k can also use an external 800k disk drive with the HD20 INIT (and
this works out well, 400k internal, HD20 hard drive, 800k external floppy).
The early Macs can support up to 3 floppy devices (including the one

Paul "neon" Gooch ( [quoted with permission]

More Apple HD 20 and related info (by permission)

To: "Classic Posts" 
From: [Paul "neon" Gooch]
Subject: Macintosh 128K answers
Date: Sun, 12 Jul 1998 08:38:01 -0500

After testing I have come up with the following:

[Removed- note about how 128K "can't" run an HD20, which is not correct.]

A Macintosh 128K cannot participate on an AppleTalk network.
- I tried various System/Finder combinations - nothing.
- When you open the Chooser on later System versions it will beep and not
- What I did get is an error stating the Mac cannot open AppleTalk. I
  suspect a memory issue.
- The AppleShare INIT will give me an error 02 on startup - It works on a
  Macintosh 512K.
- I saw no AppleTalk activity on the LAN at all - I do have some LocalTalk
  connectors with the LED lights telling if there is any traffic.
- AppleShare 1.1 states you need a Macintosh 512K or higher to operate as a
  workstation. I definitely believe this now.

Public Folder
- Works on a Macintosh 512K but not on a Macintosh 128K.
- When the Chooser is opened and Public Folder is selected, it will not list
  any other Public Folders on the LAN.
- I saw no AppleTalk activity on the LAN at all - I do have some LocalTalk
  connectors with the LED lights telling if there is any traffic.

I used System versions 2.0 - 4.2, Finder versions 1.1g - 5.5 and different
Chooser versions (I am not sure about the version numbers).

If anyone has any other suggestions I like to hear them,
The Mac 512
(note: check Herb's Mac Web pointers section to find this site)

Notes on 128K and HD20

From: Paul Grammens (grammens ATT
Subject: Re: HD20
Newsgroups: comp.sys.apple2
Date: 2002-03-14 10:22:01 PST 

Here's a little info on the HD20. Couldn't find much on the net.

It cost over $1000 new in 1986 (although I believe it was introduced
sometime in 1985). Inside is a 3.5" HH Rodime 552 disk, which actually has
an Apple Disk Drive interface right on it. I had expected some sort of
conversion circuitry (like the old Sun ESDI->SCSI boards), but there

  Average seek: 415 ms
    Rotational: 394, 429, 472, 525, 590 RPM
Burst transfer: 489.6 Kbits/sec (serial)

  Average seek: 85 ms
    Rotational: 2744 RPM
Burst transfer: 500 Kbits/sec (serial)

It looks like the 500kbits/sec is a limitation of the floppy interface and
not of any device on it.

Anyway, since the 512k only knows how to boot from the internal floppy,
and speaks only MFS (the original, flat Mac File System---the folders are
purely ornamental! Stupid trivia bit: folders on MFS disks have one extra
pixel) on a 400k disk drive, Apple got creative with their solution:

The Apple HD20 INIT (introduced with System Software 1.1) patches the ROM
to allow the use of HFS (a 20 meg flat file system would be a horrible
mess), the HD20, and the 800k disk drive.

Insert the boot disk with 1.1 and the HD20 INIT, the Mac boots half-way,
spits out the disk, and continues from the HD20. Kind of neat.

On the Plus (and presumably 512ke), which speaks HFS and 800k disk drive
natively, you can boot right off the HD20. But the Plus has SCSI, so this
is trivial."


Re: Apple 20 MB HD w/ Floppy Interface

    * From: Phil Beesley
    * Subject: Re: Apple 20 MB HD w/ Floppy Interface
    * Date: Thu, 12 Jun 2003 14:29:04 -0700 

The most intelligent discussion of the HD20 I've seen can be found at
[article from comp.sys.apple2 as copied above ] It contains a pretty good
description of what is inside the HD20 and some speculation from 
knowledgeable people about how it works.

If you find that an HD20 doesn't work, this snippet (from a net.micro.mac
 usenet posting in the 1980s) about the HD20 test utility may be useful:

"Jeff I tried it and it worked fine. For those curious, if you hit Cmd-D at the
initial dialog box on the HD20 test it then shows two windows: one has boxes
containing Block Count, Iteration, Soft Rate, Hard Rate, Comm Rate, Loops,
Total Blocks, and Failure Code. There is also a radio button labelled Dstrct
which, I believe, will allow you to reformat the hard disk. The program also
creates a document file that has a log of the tests done and the results."

The HD20 test app is on the boot disk originally supplied with the drive. 
Apple allegedly also had a couple of in-house utilities called HD Diag and
 Scavenger Mac for fixing HD20s.


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Herb Johnson
New Jersey, USA
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