Neither the IIci or (I believe) the Centris 610 need a system enabler for System 7.5.x.

Q: At any time have the Zip Drive had "The Click of Death" occur?

Q2: How old are these disks? If over 10 years old, they may have went through media failure, and it is not the drive but the disks. Same thing happens to Floppy Disks.