Here are the basic rules of SCSI for Macintosh computers:

Any SCSI device hooked to any Macintosh computer must be powered on the entire time the computer is on. When you connect/disconnect devices you must first shutdown and power off all SCSI devices.

Every SCSI device must have a unique address (ID #) for each SCSI bus. Most Macintosh computers have one SCSI bus. ID numbers must be in the range 0 through 7 (the Macintosh is 7); internal hard drives are set at the factory to ID 0 and internal CD-ROM drives are set to ID 3. "Wide" SCSI cards can support SCSI IDs 0-15, but remember that the card itself needs a SCSI ID; the normal default ID for cards is 7. You can have the same ID on different SCSI buses (e.g. you can use ID 0 on a SCSI card even if the built-in SCSI bus also has a device at that ID).

Termination must be correctly set. The beginning and end of the SCSI chain must be terminated. Internal termination is usually set correctly at the factory and therefore you needn't worry about it when adding external devices. However, some computers have special termination requirements. Check the manual for your computer for any special instructions on termination and SCSI in general. If you add external devices, you simply need to terminate the device physically furthest along the chain from the computer. Active termination is required for Fast and Ultra SCSI, and desirable for regular SCSI as well.

It is important to use good quality cabling and not to exceed the limit of 6 meters (19.6 feet) for regular SCSI. For each cabinet you can estimate a foot of cabling to add to the actual cable length (more with multi-device cabinets). Also add a foot for the impedance change for each cabinet. Fast (and fast & wide) has a limit of 3 meters. Ultra SCSI (Fast-20) has a limit of 3.0 meters for 1 to 4 devices and 1.5 meters (yes, 4.9 feet) for 4-8 devices. We have found that it's best to stay closer to 1.5 than 3 meters.

Hard drives and similar storage devices have a "driver" which was installed when the drive was formatted. The driver is used by the Macintosh computer to access the device. Other storage devices such as CD-ROM drives will have an extension or control panel to act as the driver for that device. It is important that the driver be compatible with the computer and version of Mac OS you have. The company that distributes or publishes the software can provide compatibility information.