Well, there are several flavors of IOMEGA Zip 100 drives around; but if you say, during boot your drive is recognized, it surely isn't a driver issue.

Wether your drive is "detected" as /dev/sda or /dev/sda4 depends on how the media in your drive was formatted - with or without partition table. As root user in a console you might want to try the following commands:

First create a mount point:
mkdir /mnt/zipdrive

Then try to mount the zip drive:
mount /dev/sda4 /mnt/zipdrive

If this results in an error message, try
mount /dev/sda /mnt/zipdrive

If this also results in an error message, please post it here. In case you could mount the media without error message, you should now be able to cd /mnt/zipdrive (or open that directory using the file manager) and browse the content of your zip disk. (Note: right know you can only write to your zip disk as root user.) Don't forget to umount /mnt/zipdrive before you remove the zip disk!

Next steps would be to add an entry into your /etc/fstab and a desktop icon to mount your zip drive via mouse click.

Thomas