Zip drivers are odd.

http://www.jagshouse.com/zipMacPlus.html

Look about halfway down that page under "Auto update quirk"

Therefore, if you want to not have problems with 'old driver' disks, put an old version of the iomega extension in your Mac OS 8 Extensions folder, and be certain you don't have a new version there too! (Also don't use a new version of Zip Tools.)