Seem to recall that it is done via software - not physically as with a floppy.
Some years since I last used a Zip drive, but I have a vague feeling that if you right click on the zip drive in My Computer with a disk in the drive there is a tab there -or via the Properties option.
Pete, I went to the Iomega chat line. Communicated with a tech, and after his suggesting what I had done he had me go to Start|Run type in *cmd*, OK and at the command prompt type *Format W:*. - Came up with the same response - *Unable to format, disk write protected*. Have chosen the Quick and Full format, same results.
Both Iomega disks on which I have saved photos. Other disks format fine.
A real poser!