You can also try moving files from within the web admin, going to files and media-> media files sync, expanding video, when you click on a file or directory, in the view that comes up, there is an option to move - never tried it though but saw it there.
You can also ssh to the server go the /home/public/data/videos (for example), you should see the link to you drive (most likely will show in a different colour), with the device number, and type mv *.mkv <the link to your drive>
Also you can use winscp/filezilla to connect to the core/hybrd and navigate the folders.