Since the IomegaWare software isn't compatible with OS 10.5.x, you wouldn't attempt to install it anyway. Iomega had a tech support article about this some years ago. A Zip 250 drive can read and write both Zip 250 and Zip 100 disks, but erasing/reformatting a Zip 100 disk in a Zip 250 drive with the IomegaWare software takes a long time.