Zip drives are a real pain with amigaOS.
I recently intended to use one on my second channel with IDEFix and OS 3.1. I installed everything required, have read everything readable on internet about amiga and zip drives, tried master and slave. First it wasn't recognized at all, then it appeared as DF4: allowing me to back up my boot partition, then disapeared again for never coming back whatever tweak i could make.
Now i have trashed that damn zip drive and i rely only on CDRW for backup, and i feel much better :-D
In conclusion, zip drives don't deserve any attention. They are badly recognized, work slowly and are not a safe backup solution.
May be better with SCSI Zip, but i only had an IDE one.