You cannot use any disk program with OS X with the Classic Environment. The only one that will work is $ dd through the Terminal. In Mac OS 9 on compatible machines like the Mac mini G4, the floppy drive should be plugged in first, then Disk Copy 4.2 will work with it. Disk Copy 6.3.3 will not work: use Disk Copy 6.4 or 6.5b13.