The Iomega Zip 100 external drive tends to have a built-in problem-from time to time it will identify your most important backup disk an being not formatted.
After a lot of experimentation i found a solution to this problem..one which may raise a few eyebrows...
step 1. Scream abuse toward Iomega at the top of your voice
Step 2. Remove offending disk from the drive
Step 3. Format the drive (with no disk inside)
Step 4. insert disk, run scandisk,defrag
Step 5. Beg the forgiveness of Iomega for all your abuse