If you insert a factory-formated Zip cartridge, Windows 2000 may be unable to mount or read the volume. The Disk Management snap-in shows it as being unallocated?
If the Zip drive is operating in floppy mode, factory-formated cartridges can NOT be mounted.
Take the drive out of floppy mode, using the documentation that came with your drive.
After you format the cartridge, you can return to floppy mode, if desired.