In the dosbox.bat it is not necessary to mount Just if you make some manipulation outside from the dosbox.bat file.
The drive is automatically mounted before dosbox.bat started.
You can control this very easy if you make a small dosbox.bat with just the lines
CLS
DIR/P
EXIT
or just this to see what he does, this 1 line dosbox.bat
CHOICE
Even when you are starting dosbox.bat on Z: he will display you the directory in