I have had a Mac Plus booting System 6 from both Zip 100 and 250 drives this evening. The trick is the preparation of the Zip cartridge. The procedure is similar for both types.
I used a Plus with a SCSI hard disk, placing the Zip drive as the last device on the external SCSI chain. The hard disk received a clean installation of System 6.0.8 (the AppleShare client was broken in the previous installation) with the Iomega INIT 4.2. For the Zip 250, I used an INIT that had been modified in the way described previously.
Cartridges that had previously been used in a variety of Macs were used for this exercise. It can be assumed that they had later drivers installed at the media level. Using the Anubis utility from CharisMac, cartridges were low level formatted (to completely remove the old media driver) and initialised. After quitting Anubis, the cartridges mounted on the desktop. I copied the System Folder from the hard disk to the Zip cartridge, shutdown and removed the hard disk from the SCSI chain.
I plugged in the Zip drive as the only device in the SCSI chain, and the Mac Plus simply booted. This even worked with a Zip 100 cartridge (previously formatted in a Zip 100 drive) in a Zip 250 drive. If you are booting from a Zip cartridge, the Iomega INIT does not need to be installed in the System Folder.
I ran out of time for further testing, so there are things worth trying. I don't know whether you actually need the INIT during the format process, and I'll try to repeat the tests without it. I used Anubis because I have had success using it with awkward SCSI media in the past. I turned off blind writes in the format (standard practice for a Plus or SE) but used the other defaults. Other formatting utilities should work, but you'll need to do your own homework. You don't need a hard disk to boot for the formatting, but it is quicker than using floppy disks. I used a late model Plus from 1989/1990 with third generation ROMs, so the procedure needs testing on earlier models.
I used a Plus with a SCSI hard disk, placing the Zip drive as the last device on the external SCSI chain. The hard disk received a clean installation of System 6.0.8 (the AppleShare client was broken in the previous installation) with the Iomega INIT 4.2. For the Zip 250, I used an INIT that had been modified in the way described previously.
Cartridges that had previously been used in a variety of Macs were used for this exercise. It can be assumed that they had later drivers installed at the media level. Using the Anubis utility from CharisMac, cartridges were low level formatted (to completely remove the old media driver) and initialised. After quitting Anubis, the cartridges mounted on the desktop. I copied the System Folder from the hard disk to the Zip cartridge, shutdown and removed the hard disk from the SCSI chain.
I plugged in the Zip drive as the only device in the SCSI chain, and the Mac Plus simply booted. This even worked with a Zip 100 cartridge (previously formatted in a Zip 100 drive) in a Zip 250 drive. If you are booting from a Zip cartridge, the Iomega INIT does not need to be installed in the System Folder.
I ran out of time for further testing, so there are things worth trying. I don't know whether you actually need the INIT during the format process, and I'll try to repeat the tests without it. I used Anubis because I have had success using it with awkward SCSI media in the past. I turned off blind writes in the format (standard practice for a Plus or SE) but used the other defaults. Other formatting utilities should work, but you'll need to do your own homework. You don't need a hard disk to boot for the formatting, but it is quicker than using floppy disks. I used a late model Plus from 1989/1990 with third generation ROMs, so the procedure needs testing on earlier models.