Iomega Zip mystery

I am running NT4 with Novelle . Zip disk which can quite happily be read on Win 98/ 2000 machines cannot be read on the NT4 machines unless they are re-formatted. This is even if the previous day they were successfully used on the NT4 machines. The does not seem to be a logical pattern to this although I cannot monitor how the different users sign of and remove there disks. Still they work with 98 everytime so?
Steve