I haven't used QTSS in ages, but can you enter a path of the form:
/Volumes/OtherDiskName/path/to/movies
If you didn't know, other local and network disks are mounted in the /Volumes folder, which is invisible in the Finder. To view the folder in the Finder, go to the menu item Go : Go To Folder (or shift-command-G), and enter /Volumes in the sheet.