First, make sure that you know where the end of your SCSI chain is and make sure that only that device is terminated.
I've run into the same problem on my StarMax 603e and it sporadic. One thing that helped was rearranging the order of the SCSI devices. It seems to like some better than others. Again, make sure that only the device on the end of the chain is terminated.
If you are actually tring to use the zip disk as a system drive, you may need to cold boot (start with the computer off) and hold down command, option, shift and delete when the computer sounds it initial "boing". This will bypass the internal drive and boot from the first external media the ROM sees that has a valid system residing on it. The caveat is that no other drives will mount. You'll need to use CDTools, or APS Tools, or Apple's HD Rescue utility to mount the internal hard dirve.
Good luck.