This disk can be copied without problem, but the copy don't work. The point here is to store the magic number $55 in the location $0CBD and skip the checksum routine. Boot a DOS 3.3 disk insert the copy BLOAD SHELL,A$C00 CALL-151 C51:A9 55 4C 98 0C (LDA #$55, JMP $0C98) BSAVE SHELL,A$C00,L522 The boot program must be LOADER. N.B. if you want more details search for The Rocky Horror Show, the idea is the same.