I have a directory on another drive that I want to show as a new virtual drive. How can I do that?
subst X: F:\some\folder
There is also a great freeware utility called Visual Subst to make things even easier:
I used the registry edit on wikipedia so Windows would automaticaly create the drive when starting (tested on Windows 8.1):
Another option is to use a VHD (or use a USB and skip steps 2 - 4):