This is the most intelligent discussion that I've seen. The full thread can be found at:

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From: Paul Grammens (grammens@svn.net)
Subject: Re: HD20 

View this article only
 Newsgroups: comp.sys.apple2
Date: 2002-03-14 10:22:01 PST 

"you" <you@somehost.somedomain> wrote in message
news:Gaij8.260$UI4.159383@news.uswest.net...
> >No one has cracked the lid and
> >looked at one?
> I am sitting in my shop with an open HD20 in fromt of me.  Inside we find a
> Power Supply a fan, a disk, and a board.  The disk is a Rodime Model 552.
> Connector seems to be a 26 pin affair similar to a Apple II SCSI card
connector

There is one other 26 pin SCSI that I know of, the one used in the Mac
Portable. That's an old luggable Mac. Most of them have a wire harness
soldered to a 40 meg SCSI hard disk, a few have an adapter cable that plugs
into a normal SCSI connector.

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

"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
isn't.

(400k)
  Average seek: 415 ms
    Rotational: 394, 429, 472, 525, 590 RPM
Burst transfer: 489.6 Kbits/sec (serial)

(HD20)
  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."
  

 

Message 17 in thread
 From: Wayne Stewart (waynes@intergate.ca)
Subject: Re: HD20 


Ê

View this article only
 Newsgroups: comp.sys.apple2
Date: 2002-03-14 21:41:04 PST 
Bart wrote:
> 
> >No one has cracked the lid and
> >looked at one?
> I am sitting in my shop with an open HD20 in fromt of me.  Inside we find a
> Power Supply a fan, a disk, and a board.  The disk is a Rodime Model 552.
> Connector seems to be a 26 pin affair similar to a Apple II SCSI card connector
> A DB25 (DE? ;)  SCSI  ribbon will fit here.which leads to a matching connector
> on the baord.  again a 26 pin connector.  the rest of the board is...:
> Silk Screened Apple Computer c 1985 with no Rev number
> socketed chips:  1st seems to be a Apple one  marked 620 VM
> B414...2365-1239....342-0343-B... c 85 Apple   Korea  it is a 28 pin one and it

A 2764 EPROM

> is twined by another 28 pin that is marked: 625v
> c244....VP4060-0001....344-0041-B...c Apple 1982 1983, 

The 344-0041-B is identical in part # and form factor as the IWM used in 
the Mac Plus and on the daisy-chain card found inside the 3.5" UniDisk.