Map a drive with the SUBST Microsoft DOS command

When I map a drive with the SUBST command I ensure that I have run the Command prompt as an Administrator.

Example:

subst D: C:\

If you get an error it may be that the drive may be reserved for something else (i.e. CD ROM) . To resolve this change the offending hardware to use a different drive letter. Do this by goingto Control Panel, Administrative Tools, Computer Management, Storage, Disk Management, right click on the drives you want to change and select “Change drive letter and paths”.  I found that tip from link below:

http://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/change-dvd-drive-letters-in-win-7/f1473098-a04e-4985-a6b9-04018ab4588c