Good news! I got a Mac Plus today, and the Zip 100 drive works great with it. I didn't do anything differently than I had before, so I have to conclude that the SCSI hardware in both the other systems I tried is broken. I strongly suspected there was something wrong with the SCSI on the SE/30, since it's internal HD had disappeared a few days earlier. And I'm not too surprised that the Brainstorm SCSI add-on for the Mac 512K is weirdly different enough for it to not work either.
After booting from a floppy with the Iomega 4.2 driver, the Plus immediately recognized and mounted a Zip disk I inserted. The disk was already Mac formatted, but I think it had a driver newer than 4.2, because if I attempted to boot from from the Zip I got a black screen and weird speaker buzzing! It worked OK when inserted after booting, though.
I first tried to follow the Lido 7.56 format instructions at http://www.vintagemacworld.com/pluszip.html, but the process didn't unfold the way it's described on that page. I was never prompted for anything about blind writes or ejecting removable media-- it just formatted without asking any questions. Attempting to boot from the Lido-formatted disk just showed the floppy with the question mark, like the Mac didn't recognize the disk.
I then tried the Iomega 4.2 driver method described at http://www.jagshouse.com/zipMacPlus.html. Briefly, you just boot from a floppy that contains the 4.2 driver, then insert the Zip disk, erase it using Finder, and finally copy your system folder to it. That worked, and now I can boot from the Zip drive.
I wish the Lido method had worked, however, because there's an evil "feature" lurking in the Iomega driver. The problem is described in both of the Plus Zip pages that I mentioned. If you boot up another Mac that's running Iomega driver 5.x or newer, and then insert your 4.2-formatted Zip disk, it will silently upgrade the driver on the Zip disk to 5.x. If you then bring that Zip disk back to the Plus, it won't boot anymore-- you'll get a crash on boot. I think it will still work if you insert the Zip after booting from a floppy, but the ability to boot a Plus from that Zip is gone until you reformat it. The Lido driver doesn't have this problem. With the Iomega 4.2 driver, you'll be fine as long as you only use the Zip on your Plus, or are careful to only share it among Macs that also have the 4.2 driver, but it's something I'd rather not have to worry about.
After booting from a floppy with the Iomega 4.2 driver, the Plus immediately recognized and mounted a Zip disk I inserted. The disk was already Mac formatted, but I think it had a driver newer than 4.2, because if I attempted to boot from from the Zip I got a black screen and weird speaker buzzing! It worked OK when inserted after booting, though.
I first tried to follow the Lido 7.56 format instructions at http://www.vintagemacworld.com/pluszip.html, but the process didn't unfold the way it's described on that page. I was never prompted for anything about blind writes or ejecting removable media-- it just formatted without asking any questions. Attempting to boot from the Lido-formatted disk just showed the floppy with the question mark, like the Mac didn't recognize the disk.
I then tried the Iomega 4.2 driver method described at http://www.jagshouse.com/zipMacPlus.html. Briefly, you just boot from a floppy that contains the 4.2 driver, then insert the Zip disk, erase it using Finder, and finally copy your system folder to it. That worked, and now I can boot from the Zip drive.
I wish the Lido method had worked, however, because there's an evil "feature" lurking in the Iomega driver. The problem is described in both of the Plus Zip pages that I mentioned. If you boot up another Mac that's running Iomega driver 5.x or newer, and then insert your 4.2-formatted Zip disk, it will silently upgrade the driver on the Zip disk to 5.x. If you then bring that Zip disk back to the Plus, it won't boot anymore-- you'll get a crash on boot. I think it will still work if you insert the Zip after booting from a floppy, but the ability to boot a Plus from that Zip is gone until you reformat it. The Lido driver doesn't have this problem. With the Iomega 4.2 driver, you'll be fine as long as you only use the Zip on your Plus, or are careful to only share it among Macs that also have the 4.2 driver, but it's something I'd rather not have to worry about.
