By default, SMARTMon will not poll disk devices with removable medium. If you have an Iomega zip drive, for example, and this option is enabled, then SMARTMon will generate a device not ready message every polling interval, if the cartridge is not inserted in the drive.
Note that is field is applicable to disk drives, and not CD-ROMs or CDRWs. SMARTMon never automatically polls these devices anyway.