The forward slash on D:/ is incorrect, should always be the back slash \ when working in File Explorer/Windows Explorer/Command Prompt. The Internet makes use of the forward slash / in URLs. Also, running a command in Command Prompt that can have switches will use the / to denote the switch.
D: is the letter for a partition on the HDD or could be a plugged in drive. Right-click the Start button, choose Disk Management and check what partitions or drives you have. If D: is actually a second HDD could be it is failing or has failed. If it were mine and the problem exists after a full shutdown and startup I'd run chkdsk /r /f to check it. http://upgradenrepair.com/Instructions/C/chkdskr.htm
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