Oh god, was it that simple! A little backslash! I feel stupid 😢

I have two HDD's on my computer ( C: and D: ). Mounting the c: disk works fine, but when I try to mount the d: disk it gives me an error message (Directory d: does not exist). The code I'm using is: mount c d: or/and mount d d: Can you tell me what I'm doing wrong? Thanks!