You cannot assign a driveletter with subst unless it already has a driveletter.
If you want to assign a drive to a portion of an existing driveletter, you can do so from Computer Management.
- Go to Start
- Click Control Panel
- Go to Administrative tools
- Go to Computer Management
- Go to ComputerManagement (Local)->Storage->Disk Management
- Choose the partition that you want to be accessible
- Right click the bar and choose
Change Drive Letter and Paths
- Click Add...
- Click the Browse... button and navigate to a path, create new folder and then OK to create a virtual path to this volume. For example: Volume1 points to: c:\other_hdd
- Press OK
If you now remove the driveletter to this volume, you can still access the content of the HDD through the path you created in step 9, for example: C:\other_hdd.
You can now use Subst to link a driveletter to a subfolder on that volume, for example:
subst e: c:\other_hdd\myfolder
Of course, it does mean that using c:\other_hdd your old harddisk is still accessible, and there's no way to prevent that. If you really want to make that place not being accessed, consider
security by obscurity and hide the link somewhere deep inside a nested subdirectory tree where people are likely not to look.