Iomega zip the disk is not formatted err

Hi,
I have a 100mb zip disk that has data on it. I was able to access the data before. now when I try to access the disk it says The disk in drive X is not formatted. Do you want to format it now.
The disk is filled with data. I have tried it on another zip drive. I can use other disks that have similar data on them. I have used the iomega software and removed the write protect.
Any Thoughts
thanks