Cookie,
The SCSI chain was a lot of power provided to a home computer. It contributed to the higher cost of the computer. It also added to the longevity of the hardware usefulness and ease of use. It was decades before a PC could boot from an external device the way a Mac could in the '80s.
Here is what matters in SCSI.
1. It is not plug and play until configured. Each device has to be configured to a specific address. The benefit is the ability to have seven devices recognized at startup.
2. Termination. The signal needs to know when to stop searching. At startup, the host or computer will 'poll' the devices. The first device that it sees with a system folder will allow the computer to boot up. With 'Startup Disk' you can reassign that task to another system folder on another device. The benefit? You can easily repair one hard drive by booting from another drive. Termination tells the host that all devices have been accounted for and it can now move on to the task of reading a drive. Zapping the PRAM tells the host to start fresh and poll all devices. It takes longer to startup but it creates new directories, or table of contents, so to speak. Otherwise the computer may lose track of where to find a file.
3. The highest ID number just sets the priority or que during the device access. The higher the ID #, the sooner it comes in the polling process.
If you want to optimize your SCSI devices, study the rules programmed into the protocol, otherwise, just enjoy the fact that a Mac will let you swap out drives and reconfigure without a lot of trouble. A 'D' drive in a PC will not boot the computer, even if it is a carbon copy of the 'C' drive. In fact, change the C drive to the D drive location in an older PC and the PC will not start.
The external SCSI port was the greatest hardware tool Apple ever offered to the government surplus shopper. We would buy Macs at government auctions and test them within minutes with an external zip drive or hard drive. The PC guys would spend a day on each used PC because they had to find drivers for the video card, the network card, the hard drives, etc.. The SCSI chain was plug and play if we had our external drive set at ID 1 or our zip drive set at 5 because the internal drives were almost always still at the factory setting of 0.
Enjoy your mac and good luck with your optical drive.