Also, maybe worth mentioning.. My Zip 100 Plus drive has two outputs in the back. One is a "AutoDetect" connector, the other seems to be a regular SCSI output connector, with a switch between SCSI ID5 and ID6. I connected the Mac to the regular SCSI connector and tried both SCSI IDs.

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If your HD won't boot when the Zip is connected and there's not a SCSI conflict, then there can only be a termination issue. THere should be a switch for term power on/off. Also, the auto-detect is most likely a parallel/SCSI hybrid. It auto-detects which you are connected to. The other SCSI port should be the pass-through ONLY. So, use the auto-detect port, select 5 or 6 (should not matter if nothing else is connected) and set term power off. Use alternate settings if that does not work. Assume you have plugged in the Zip drive to external power supply?