>Can a 250meg zip disk be used in the 100meg drive? Does the disk need to be reformatted. Can the 100 meg read/write all 250 megs?

No, I wouldn't think so. A 100MB cartridge in a 250MB drive should work, but not a 250MB cartridge in a 100MB drive. (I currently do not have a 250 drive or cartridge to verify this, FWIW)