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SCSI Voodoo

SCSI Voodoo

Your Mac can be connected to external devices such as hard drives, removable cartridge drives (Zip, Jazz, Syquest, optical), tape drives, and scanners using SCSI cables. SCSI is an abbreviation for Small Computer System Interface. SCSI is an excellent way to connect computer peripherals. It is used as a standard by high-end UNIX workstations (Sun, Digital, and Silicon Graphics machines) as well as an add-on option for IBM-compatible PCs. However, it is not quite as a simple and user friendly as most Mac technology.

SCSI devices are connected in a chain from a single connector in the back of your Mac. Each SCSI device has two SCSI connectors, one to connect to the Mac (or to the previous device in the chain) and one to connect to the next device. You can have up to a total of seven devices linked one to another on a single SCSI chain or "bus", but your Mac already uses some of these spots for internal devices. Your hard drive itself is (probably) a SCSI device, as is any internal CD-ROM drive, and any other internal drives (Zip, second hard drive, etc.). This means if you have an internal SCSI hard drive, and a CD-ROM drive then you can only have five additonal SCSI devices connected to your Mac.

SCSI Rules

There are 3 basic rules that govern the set-up and troubleshooting of SCSI chains.

  1. Every device must be assigned a unique SCSI ID number (from 0 to 6). This is usually done with a small switch on the back of the device. If two devices have the same ID number, you can see all sorts of problems and crashes. Note, there is a SCSI ID#7, but this is always used by the Mac's SCSI controller device and is not available for any other purpose.
** There are some standard conventions for numbering. ID 0 is normally used for the primary hard drive, while ID 1 is used for a secondary HD. ID 3 is used for CD-ROM drives. ID 5 is common for Zip drives and ID 6 for scanners or tape drives. Keeping to the conventions can help minimize problems as some devices come pre-configured.

  1. The last device on a SCSI chain must be "terminated". Some devices have internal termination that can be turned on with a switch. Otherwise a small terminator "plug" can be installed in the open connector of the last device in the chain.

  2. The overall length of the SCSI cables from your Mac to the last device on the chain must be less than 18 feet, including the wiring inside each device and inside your Mac.

There are also some guidelines you should keep in mind, but we don't consider them rules because you can find exceptions and differences of opinions out there:

  1. Never connect or disconnect SCSI devices while the computer is turned on you may cause severe damage to your machine or SCSI devices.

  2. All SCSI devices connected to the machine should be turned on while the computer is in use. Lots of people ignore this guideline, until they have problems. (Kim often leaves the scanner off until she needs to use it.) Some computer/device combinations are tolerant enough to let you get away with it, but not always. So watch out!

Setting up SCSI devices on a Mac

There isn't a strict rule about the order in which to hook up SCSI devices in a chain, but there are some guidelines. Fast devices that transfer a lot of data should be put closest to the Mac in the chain. Scanners and tape drives like to be last in the chain.

Each SCSI device must have a "driver" software that controls its operation in your Mac's Extensions folder (in the System folder). For example, the Iomega Zip drive uses the Iomega Driver.

Here is a basic procedure to follow for adding a new SCSI device to a Mac.

  1. Check the SCSI ID#s that the Mac is currently using. You will need a utility program such as "SCSI Probe" or "FWB Hard Disk Toolkit". FWB Hard Disk Tool Kit window
  2. Set the SCSI ID# of your new device to a number that is not being used. Look for a small switch on the back of the device.

  3. Hook up the cables. Be organized and methodical go from your Mac to the first SCSI device...from that to the next...and so on. Use the shortest cables that you can find remember the overall length of the whole chain must be less than 18 feet.

    Invest in high quality cables double-shielded, twisted pair cables are a must. Don't let anyone try to sell you PC-style parallel cables to use on SCSI devices cables like these could cause major SCSI problems. Vendors like APS and Apple sell well-made cables you can count on.

  4. Make sure that the last, and only the last, device in the SCSI chain is terminated. Again this is via a switch on the back of the device or by a special adapter inserted into the open SCSI connector.


SCSI problems can cause a wide variety of symptoms on your Mac, from "Sad Macs" or flashing ?  at boot to erratic disappearance of disks from the desktop. However, once set up correctly, a SCSI chain should not suddenly begin to malfunction.

If a SCSI device is not working:

  1. Re-check all power and SCSI cables.

  2. Remove all other SCSI devices and connect the problem device directly to your Mac.

  3. Check that the correct driver Extension for the SCSI device is loaded in the Extensions folder (and checked as activated in the Extensions Manager).

  4. External hard drives and most removable cartridges must be "mounted" so that they appear on your Mac's desktop like a floppy disc or hard drive. This is generally done automatically by the driver Extension. However, some devices (and some cartridges) need a bit of help in order for the Mac to recognize and mount the device. A SCSI utility program like SCSI Probe or FWB Hard Disk Toolkit will be helpful if your Mac is having difficulty mounting a SCSI device.

  5. For removable cartridge drives like Zip or Syquest drives, there is the possibility that you have a bad cartridge. Try another cartridge and/or test your cartridge on another drive mounted on another computer.


SCSI is moody. You will occasionally find that each SCSI device works fine when connected by itself to your Mac, but there is a malfunction when a group of them are chained together. This is where the demons of SCSI Voodoo begin to haunt your computer.

It takes some fiddling to get the SCSI chain right and it sometimes feels like you have to appease the SCSI gods before it all works smoothly. Just be methodical in your set-up and troubleshooting. You will be able to get away with incorrect SCSI configurations from time to time, but they normally catch up with you, particularly when you move to a new shiny faster machine. The faster the machine is, the less tolerant.

First re-check the rules: ID#s, length of cables, termination. Then try tweaking the set up. Change the order of devices in the chain or try switching ID#s. Finally check for Extensions conflicts that involve SCSI drivers.

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