How can I secure/encrypt a file held on an external HD

I would like to put items on my external wireless networked lacie HD which cannot be accessed by others who use it. I thought creating a disk image would work but after putting the files and transferring to the public folder on the HD, I want to delete the files from my local drive as they take up so much room. When I then log back into the Lacie the files I put there have disappeared. I've re-created this a few times and it seems that the image on the network drives mimics what I do locally.
Am I doing something wrong? I know how to set encryption and unchecking the pswd in keychain.

Thanks
Stuart