SD Vs CCC

[Andrew]

IF I am copying my primary system HD to an external HD, can you make an exact copy of the entire contents (active programs and operating system included) from the drive you are running SD from? I had been told that copying programs like SD or CCC cannot copy the active programs and OS while they are used. I was told one has to boot up from a third drive to copy drive one (the primary system HD) to a second drive. So you really have to use a method of using 3 drives where A would have the operating system, and you would be copying drive B to drive C. I am confused.