Wai,
My zip drive problem was explained by Wil Davis, who also posts in this forum. I emailed him for help, and he promptly responded with the answer. I will share with you what he said:
First - make sure your Zip drive is a SCSI drive (assuming yours is an IOMEGA Zip drive) - the parallel-port version looks very similar, and has almost identical connectors (although they are two females on the SCSI version, and one female and one male on the parallel-port version - the parallel-port version doesn't have the terminator and id-selection switches). The parallel-port version WILL NOT WORK ON THE SCSI INTERFACE!!!!!
According to your description, you only have the Zip drive on the SCSI chain, so the SCSI address of the Zip drive isn't likely to be in conflict with anything else on the chain, however you might want to change the address - just to see if it makes a difference (if you have an IOMEGA Zip drive, there are only two choices: 5 or 6).
The main thing is to make sure that the SCSI cable is OK (and not too long), and that the cable is terminated correctly at the Zip drive end (if you're using an IOMEGA Zip drive, there is a terminating switch (on/off) which needs to be in the "on" position if the drive is at the end of the chain).
Hope this helps -
- Wil