Ext HDD, adding things to it from the computer?

My Ext drive is showing up on the ATV (menu) under Files/Places, but the big question for me is, How do I get stuff onto it from my computer (Apple), do I do it through the FTP client somehow?? I don’t want to use it as the primary storage,just extra space, or do I have to do this??
Can someone help me with this please.

If you look under Movies in your home folder you should see a shortcut to the external drive.

I can see the HDD on the menu under places, at the moment the folder is empty because I haven’t put any movies/files on it, that is my question, how do I get files on the HDD from my computer. I am using Cyberduck FTP, it doesn’t show up on it, I know if it did I could just drag files to it (the HDD). Help please??