I too had a problem with my iomega zip usb 100 drive and mac os 9. Whenever I tried to access the zip drive options my imac would freeze. Sometimes when I turned on my imac it would freeze. I was beginning to wonder if I had wasted my money with iomega.
The solution, however, was found at www.versiontracker.com
where I downloaded the iomegaware 2.1.1 version update. Using the keyword 'zip' I followed the link to the page where iomegaware 2.1.1 was listed, I downloaded it, installed it and now my iomega usb zip 100 drive works fine with no problems at all alongside mac os 9. No more freezes, no more startup problems. It works like a charm. I moved the old iomegaware folder, which was installed from the CD, it was version 1.1 by the way, to the trash.
It's a shame that iomega does not mention this info on their website. With all that I have read about mac users experiencing many different problems with the compatibility of mac os 9 and iomega zip usb 100 they would post news or info or an update on their website.
Anyway mac friends, if you have had the problems I have had with mac os 9 and iomega zip usb 100, just download the iomegaware 2.1.1 update and install it and there should be no more problems with compatibility.