Booting a Mac Plus out of an external SCSI2SD card

Have been working on my Mac Plus for a while, and I wanted it to be able to boot from an external SCSI drive. Up to now, I was using a Iomega Zip Drive (after sweating a lot to get it to work with the plus), but Zip Drives are not really reliable (specially an almost 25 years old one).

So, searching on the net, I realized many people complained about the inability of the Mac Plus to boot from an external SCSI2SD. On top of that, the fact that the Mac Plus is one of the few Macs that doesn’t provide SCSI bus termination power forced the external SCS2SD card to be powered by an external power source (i.e. USB or something else).

Looking further, I found some old documents and discovered that it was possible to add SCSI Bus termination power to the Plus. A very simple mod allowed for that. In fact, the ONLY thing needed is the addition of a diode to the logic board, right behind the SCSI connector. Funny thing is that Apple, apparently, accounted for that in the past, as the diode “place holder” already exists in the board (position CR1, as highlighted in red in the picture below).

So, I took a deep breath and decided to attempt the mod. In the picture below you can see the diode soldered to the place holder. I used a diode recovered from an old power supply I had. Its markings were UF2010 (UF stands for ultra-fast). It supports up to 1000v, but of course, in our case, only 5v will be crossing the diode. Be careful to solder the diode following the markings. Diodes allow for current to flow through one direction, but not the other. Adding it in the wrong direction would create issues. You don’t have to use the same diode I used. A more common one – such as a 1n4001 – would do just fine. Just make sure it takes 5v or more and about 1 amp (or more).

I wasn’t much confident this mod would work, as it was way too simple. But it did! In the video below (sorry about the quality, still learning how to create good videos) you can see the solution fully working on a Mac Plus.

7 thoughts on “Booting a Mac Plus out of an external SCSI2SD card

  1. After the scsi termination modification, what versions of SCSI2SD works best with Macintosh Plus?

    • James,

      V5.0, V5.1 and V5.5 will work equally as well, although you’ll obviously have to adapt from DB25 to IDC50, in the case of V5.0 and V5.1; They’re all equally capable of being powered via SCSI termination power. V6 would alsowork, but be a waste of money, as the SCSI controller in almost all 68K Macs can’t really make use of what the V6 is capable of, throughput-wise.

      • Hey James, sorry I took so long to come back to you! It seems Alex have just answered your question! Tks Alex!

  2. Brilliant! I took the plunge today and strapped a diode across CR1 with the stripe facing the db25 connector, et voila! I was also expecting some boot drama, but with a SCSI2SD v5.5 Pocket Edition, and the suggested settings, I had zero issues and it booted right up! A completely silent Mac Plus with an ssd has been a dream of mine for decades… than you so much for this post!

  3. Thanks for your help in posting this. A friend of mine and I also got his Mac Plus working with the SCSI2SD 5.1 board with the 4.8.4 firmware. We didn’t do the diode mod, but were able to power the SCSI2SD using the power supply in his old HDD’s chassis. One odd thing, though – the SCSI2SD will only boot if it’s powered up AFTER the Mac Plus displays the disk with the blinking “?” icon. If we have the SCSI2SD powered up before the power on the Mac Plus, it doesn’t boot. I imagine this is some sort of configuration issue with the SCSI2SD board. Would you mind sharing your SCSI2SD configuration and firmware version that you used? Thanks!

    • Hi Judd, my SCSI2SD configs have nothing special at all. As far as I remember, I am actually using the defaults there.

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