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[nitro]

Your Z drive in dosbox is not a real drive. It is only a virtual drive.

Before you can do something you should mount a directory in Dosbox with the mount command. the first argument for this command is the drive letter and the second argument the directory you wan to mount.
for example
mount c c:\mydosboxdrive
where c:\mydosboxdrive is a directory on your real harddrive in windows.

Now you can go to your new drive and create a directory there