Subst — windowssystem32subst.exe
Subst creates a new drive letter and actively links it to an existing folder on your hard disk. For example, type:
subst m: c:downloaded musicoldiesmetallica
to create a new drive letter, m:, and link it to
the folder, c:downloaded
musicoldiesmetallica. When you open drive
m: in Explorer, you'll see the
contents of the linked folder, useful if you access a particular
folder frequently but find Windows Shortcuts too limiting. For
example, a drive created with Subst allows you to access a file in
the folder, like this: m:astronomy.mp3. To
disconnect a Subst'd drive, type subst z:
/d
.