The DOS command subst is used to assign a drive letter to a folder.
For example to use a drive letter M to access the folder c:\DATA you would enter
subst m: c:\DATA
If you want to make this assignment survive a reboot then you can modify the following registry key:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices]
and add a new string value
key M: value \??\c:\DATA
If you’d rather not mess around in the registry, you can use the Persistent SUBST command from
https://code.google.com/p/psubst/
or Visual Subst available from