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Mac II HD20 Support


Well-known member
I'm continuing my old disc spelunking (can you tell?) and found another oddity on the 1992 Nordic support disc: http://macintoshgarden.org/apps/apple-nordic-support-cd-1992

This might be old news, but I wasn't aware there's an official (unofficial) extension to add HD20 support back to machines with no support in ROM. No, not the startup file for early Macs, an extension for newer systems including the Mac II:
Macintosh II HD-20 Support

This software is not supported. Additionally, the hardware and even the manual (this document) is not supported. It is not a product, and is to be used at the risk of the user. Caveat Emptor!

Macintosh II HD-20 Support

! Warning:
Since the Macintosh II internal IWM port is not buffered (as external IWM ports are in other Macintosh products); there exists a real danger of causing severe damage to the IWM itself when installing. Taking the precautions recommended here minimizes the risk of this happening, however some damage may still occur since the Macintosh II was not designed to support the HD-20. Please read all of this document before even attempting to install any of the hardware. The greatest danger is in installation and removal. At the very least, please keep the number of times that you install/remove this hardware to a bare minimum

• Turn both the Macintosh II and the HD-20 off. This step is paramount in avoiding costly repairs to both machines.
• Remove the cover of the Macintosh II.
• Touch the power supply in an effort to remove static electricity from your body.
• Remove the floppy drive cable (if your MacII is so equipped) from the internal IWM port #2 (the connector nearest to the NuBus slots).
• Connect the ribbon cable to the IWM port #2.
• Feed the ribbon cable out the rear of the machine (through an open slot access port).
• Replace the cover on your Macintosh II.
• Remove the screw and top cover of your HD-20.
• Touch the power supply in an effort to remove static electricity from your body.
• Remove the cable coming into the HD-20 at the 20 pin connector (this looks amazingly like the Macintosh II's IWM port connector).
• Connect the ribbon cable to the 20 pin connector inside the HD-20.
• Leave the HD-20 in a position where it will be least likely to cause accidental eloctrocution and where the fan is least likely to get external objects (your fingers) in it.
• Turn on the HD-20. (continued on next page)
• Turn on the Macintosh II.
• After booting the Macintosh II, insert the floppy drive to drive #1 (located at the far right hand side of your Mac II) and copy the file "MacIIHD20" to your system folder.
• Select "Restart" from the Special Menu.

Your Macintosh II will now reboot mounting the HD-20. You are now in a position to copy data from the HD-20.

I wonder if this works with later machines that still contain external floppy ports without all the fiddling.

I've attached it but I got a little lazy, it's really a Compact Pro archive and it's got some random ethernet tools stuck in there too. Sorry.


  • hd20.sit
    23.6 KB · Views: 3


Well-known member
Interesting! But I fail to understand what the risk of hardware damage is. They make it sound like after connecting an HD20 but prior to installing the extension, you would have some bus fighting or short circuit on the disk I/O signals. As far as I'm aware there is no risk. If there were a risk, it would happen again every time you reboot before the extension loads, or every time you boot with extensions off.


Well-known member
My read on it is that they were worried about ESD damage. I’m sure the external ports have ESD diodes to protect the SWIM, while this approach would be a direct, unprotected connection to the SWIM. Fine if you’re careful, and don’t hot-plug anything. Not so fine otherwise…