Yes I'm not quite sure what problem you're having other than storage, so with that in mind I'll pass along some tips etc. and hopefully I'll have elucidated well enough that the information I provide will be sufficient to answer your questions and concerns.
Simply adding another movie 'source' within XBMC using add source should allow you to point at the new drive letter (and sub folders if you have them), configure your scraper and you're good to go. Using windows O/S 7 & 8 under 'libraries' (not to be confused with XBMC libraries) under a 'movie folder' you can have multiple folders, so all areas show up (great for maintenance) but has nothing to do with the program. XBMC launches from an icon on your desktop, and has within a built-up library of your movies from the source you're specified previously. If the path to those movies is intact, then all is good. You can change paths to external drive(s) or just add them to an existing collection, but you will have to re-scan any new source, and if you are just moving (not just copying) some of the movies to the new drive, it will appear in the library as if you have two listings for those movies you have moved (a system clean-up will clear-out those duplicates). If you decided to move most movies to this new external drive, it would be wise to export your library in settings/video to separate files which will allow for a re-scan locally (quickly doesn't need to exit to the web for the same data).
In eventuality If you intend a large collection, it might be a good idea to use sub folders within the movie folder so that you're not dealing with 1000 movies in one directory listing, it would take too much time to read and make XBMC appear sluggish. Just listing a directory in windows should give you a clue as to how many files can be in a folder and still have quick response.
So you've added an external HDD to a windows system (I'll assume USB connection) and you now have a new device drive letter auto-configured. Using windows you should see something like drive C: and Drive D: great, but not all is perfect for windows plays musical chairs with your drive letter if you should boot with more than one external device (e.g. USB stick) then your drive letter might be labelled F: instead of D: which for most PC users would not mean a lot; but for the XBMC library your path to the files might not be available. The solution here is to watch what drives are plugged in on boot and try and keep this drive letter assignment the same.
I find the Wiki's on the top of this page invaluable information
http://wiki.xbmc.org/index.php?title=Fir...Installing
Here's another from an outside source
http://lifehacker.com/5536963/the-ultima...dia-center
