Sorry for the delay in answering.
On My Setup I have 2 Drives both with a folder called "Films eg S:\Films\ and a New one R:\Films\
O/K for a start CREATE a NEW folder in the directory where all your existing films are in my case I created a folder called "Films on R Drive" so I have S:\Films\Films on R Drive\
Right Click on your "My Computer" Icon on your Desktop
Left Click "Manage"
Left Click "Disk Management"
In the list of Hard Drives disk 0, disk 1 etc Right Click on your NEW Drive (my example you would Right Click on the R Drive).
After a Right Click select "Change Drive Letter and Paths"
Click "Add" and browse to the New folder you created above ie browse to S:\Films\Films on R Drive\
Click O/K
Click O/K
If you have set it up correctly you should see 2 entries entries when you Right Click on your NEW drive under "Disk Management"
I Have 1) R:
and 2) disk name (S:) \Films\Films on R Drive
Good luck
Paul T