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Mac Plus and Zip Drives Revisited

Difficulties when using a SCSI Zip drive on the Mac Plus have acquired almost mythical status. The reality is that Zip drives can be tricky to use initially on a Plus. But once you get the configuration procedure right, they provide a cheap, reliable and effective hard disk alternative for any compact Mac with a SCSI interface. The trick is to use the right tools to format the Zip drive -- don't use the tools provided by Iomega unless you wish to explore the problems encountered by other vintage Mac enthusiasts. The same or similar technique can be used to create bootable Syquest, Jaz and other cartridges.

Myth Number 1: "If you use a Zip cartridge that has been prepared for use on a Plus in a later Mac, the media level driver will be updated. This will render the cartridge unreadable on a Plus." False: This problem only occurs if you use Iomega's formatting utilities.

Myth Number 2: "You can only use a Zip 100 drive with a Mac Plus." False: You can use Zip 100 and 250 SCSI drives with a Plus.

The How To below explains how to configure a Zip cartridge that can be used safely on any Mac. A suitably formatted cartridge can be used to boot a Mac Plus or used as a data/application drive. The procedure has been tested with both Zip 100 and 250 drives. The terms "cartridge" and "removable disk" are used interchangably in this article.


  1. Mac Plus or earlier compact with a SCSI interface (128KB ROMs and 1MB RAM assumed). You will need a hard disk, external floppy disk or file server volume from which the disk formatter can be run. This is not a "bare metal" procedure; you need to be able to boot the Plus already from floppy disk or hard disk with System 6 or System 7.
  2. Zip drive and suitable SCSI cables, terminators etc
  3. Lido 7.56 disk formatting utility (Lido is free and commonly available)
  4. Iomega extensions and INITs: you don't need them and you shouldn't use them

Mac Plus Setup

For convenience during setup, a Mac Plus with a SCSI or HD20 hard disk is recommended. However, the format procedure will also work with a Plus that boots from a floppy drive.

If using a SCSI hard disk, set the SCSI ID for the existing hard disk to 6 and the Zip drive to 5. The Plus tries to boot from the drive with the highest SCSI number, so you will have problems immediately if you do not get it right. Make sure that the SCSI chain is correctly terminated. Remove any non-essential devices (eg scanners) from the SCSI chain.

I prefer to put the Zip drive as the last device on the SCSI chain and enable the switch to provide termination. This is just personal preference, but it reduces the number of cables that need to be unplugged and replugged during testing.

Don't insert a Zip cartridge immediately. If there is one in the drive already, disconnect power and SCSI. Reconnect power and press the eject button on the front of the drive.

Remove any Iomega INITs or extensions from the System Folder. Utilities such as SCSI Probe should be harmless, but remove them for this exercise because you don't need them and they are a mental distraction. Reboot after changing the System Folder configuration.

I have not tried this exercise using an HD20 as the boot drive, so you are on your own.

Zip Cartridge Formatting

  1. Boot the Mac Plus with the Zip drive connected and powered on.
  2. Start Lido 7.56. If you are running System 6 with 1MB, you may need to trim your INITs, Desk Accessories and Fonts beforehand to run Lido.
  3. When Lido displays the full user interface, insert the Zip cartridge. Wait twenty seconds or so until disk activity slows down. Press the Scan Bus button. The cartridge will be recognised, even if it it has been used previously on a Mac or any other computer.
  4. Select the cartridge in the central pane of the Lido window. Press the Format button. When prompted, confirm that it is OK to delete all data. When prompted for further formatting information Disable Blind Writes -- for data reliability, blind writes should not be enabled on removable drives, especially on a Plus or SE. The option to Eject Removable on Restart can be configured according to preference. Other formatting defaults are "safe" and can be ignored. Lido will perform a low level format, which takes about ten minutes. The low level format will overwrite everything on the cartridge, including any partitioning and driver data from previous usage.
  5. On completion of the low level format, select Eject from the menus and then Quit Lido.
  6. Reboot the Mac. Formatting utilities load drivers into RAM to support different SCSI devices. It is best to clear them out before proceeding further.
  7. When the Mac has rebooted, reinsert the Zip cartridge. After the low level format, the Mac Finder will recognise the cartridge as a brand new disk. When prompted, confirm that it is OK to format the cartridge. This will perform a high level format and the cartridge will mount on the Finder desktop.
  8. Drag your System Folder from the hard disk or floppy disk onto your new Zip cartridge. This will create a basic Zip bootable drive.
  9. Verify that you can boot from the Zip cartridge. If necessary, remove the hard disk from the SCSI chain.
  10. Shutdown and (if necessary) reconstruct your SCSI chain to boot from the hard or floppy disk. Test whether the Zip cartridge can be inserted and injected on the fly when using another boot disk.

If you are using a minimum specification 1MB Mac, you will not be able to use any "Auto Format" options offered by software because you will run out of RAM during the process. The procedure above is slower but it works.

Frequently Asked Questions

Q: What are drivers?

A: In the Mac world there are two sorts of driver. There are media/hardware drivers (hidden information on a disk or a ROM on an expansion card), and system software drivers (INITs, extensions, control panels).

Media/hardware drivers identify devices when the Mac starts up. For a disk drive, the media driver is essential if you wish to use it as a boot drive. For a video card, an appropriate ROM is required if the attached monitor is required to display the boot process. System software drivers change behaviour of devices after the Mac boots. The PC Exchange family of control panels, for example, provide a driver that allows the Mac to mount FAT or FAT32 disks which do not have a media level driver.

Q: Why does the Iomega software cause problems?

A: When you attach a hard or removeable disk to a Mac, the driver installed at media level is loaded into RAM on startup or when it is recognised by the Finder. Generally, system level drivers are welcome because they can over ride the media driver, adding extra functionality. Unfortunately, Iomega's system level drivers insist on updating the media level driver without warning, and the later media level drivers are not compatible with the Mac Plus. This problem does not occur if you use a non-Iomega utility to format the removable cartridge.

Q: Why does this format method work?

A: You are just putting a better media level driver on the Zip cartridge. Iomega wrote adequate ones, but most third party formatting utilities have better ones for removeable disks/cartridges. Iomega's system level drivers will not write over a third party media level driver on a Zip cartridge.

Q: I inserted my Zip cartridge in a drive connected to a Mac running a later operating system. Why doesn't it boot my Plus any longer?

A: The System Folder on the Zip cartridge has been clobbered by the later OS. Boot your Plus from a System 6 or 7 disk, and rebless the System Folder on the Zip cartridge. To bless a System Folder, open the folder (whilst it does not have a special folder icon) and then close it (and the special icon has miraculously reappeared).

Q: Will this Zip format technique work with other formatting utilities?

A: Yes, I have used SilverLining Lite v1.05 and an early version of CharisMac Anubis. Try your own favourite formatter, as long as it understands SCSI 2 and/or SCSI Manager 4.3. If you use a very old formatter, you may find that the media level driver that is installed does not support on the fly disk ejection and reinsertion. In the long term, you should switch to a more modern formatter, but you can use a utility like SCSI Probe to overcome the problem temporarily.

Q: When do I need to use INITs, extensions and control panels provided by a removable disk supplier?

A: You only need to use them if you have used a formatter that puts a vendor-specific media level driver on the cartridge (and the INIT etc is only required on the floppy or hard disk that is used to boot the Mac). If you boot from a removable cartridge, there is no need for an INIT etc. Special INITs are not required if you use a suitable third party formatter.

Q: Can I use my Zip cartridge to boot older operating systems than System 6?

A: I have not tested my procedures with anything older than System 6. I expect them to work.

Mac128 has tried various system combinations using Zip 100 drives on older Macs using the Iomega software. If you want to use a Zip 250 using the procedure described by Mac128, make a copy of the Iomega 4.2 INIT and rename it. Open up the copy in any binary editor on a Mac and change the references to "Zip 100" to "Zip 250". Boot using the new INIT and reformat your Zip 250 cartridge.

Q: Can I use my Zip 100 cartridge in a Zip 250 drive?

A: Possibly. Big (250) into small (100) does not work. Small (100) into big (250) works, but disk writes are very slow and low level formats are impossible. Given that both devices and media are commonly available cheaply, select the format (100 or 250) that you can buy most cheaply.


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This page last updated: 28 October 2007

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