Holly:

I need a little more information. Disks inserted into a Mac II should not be rendered unreadable by SEs. Was the entire disk unreadable (did it make a face and spit it out) or were just some of the files unreadable? Were you using MacWrite (there are some compatibility problems between the MacWrite 5.0 often found on Mac II's and version 4.5)? Did you reformat the disk while on the Mac II? It's possible that one of the two disk drives is out of allignment.

Maybe Robert has some theories to offer...