System level drivers (INITs, extensions) are not required for hard disks or SCSI removeable drives (Bernoulli, Zip, Syquest). System level drivers provide additional functionality (on the fly ejection and re-insertion, for example), but a Mac should recognise any SCSI removeable drive if it is present on startup. If that fails, use SCSI Probe or a similar utility, and remove all Iomega INITs or extensions. Any decent third party disk formatter will operate on a Zip or similar removeable disk. Formatters put a "driver" on the disk but that driver is different from a system level INIT or extension.
Zips are more awkward to deal with. Iomega presumed that they knew more about SCSI disks than others, and the system level driver (INIT) looks at the disk to check whether it has been formatted in the approved fashion. Thus a Zip 250 cartridge will not work with the System 6 INIT because Zip 250s did not exist at the time. If you want to persevere with the System 6 Iomega software, open the INIT in a binary editor and change the references to "Zip 100" to "Zip 250". Or save time by removing the Iomega INIT and by using a third party formatter (System 6 and 7 compatible!) for your Zip disks alongside a SCSI control panel.
Zips are more awkward to deal with. Iomega presumed that they knew more about SCSI disks than others, and the system level driver (INIT) looks at the disk to check whether it has been formatted in the approved fashion. Thus a Zip 250 cartridge will not work with the System 6 INIT because Zip 250s did not exist at the time. If you want to persevere with the System 6 Iomega software, open the INIT in a binary editor and change the references to "Zip 100" to "Zip 250". Or save time by removing the Iomega INIT and by using a third party formatter (System 6 and 7 compatible!) for your Zip disks alongside a SCSI control panel.