did you install the software that came with Zip drive?
don't. and if you did, disable/delete/nuke it.
really. MacOS 9 installs a driver that by all accounts i've read will work... Iomega's driver that comes with or has been released since you bought the Zip won't. smooth, huh?
i'd most likely be getting the attention of Iomega if i were a Zip user (i gave mine to my sister several months ago... one less headache) and complain as constantly and as nicely as possible.
you can research this at this forum by using the search engine and you should go to the MacNN homepage and search there too...