Losing disk space on computer when I transfer video from tivo to tivo desk top then to an external hard drive?

I just bought an external hard drive, and I've been transferring tivo recordings onto it using tivo desk top. I've been moving them to the hard drive so they are no longer listed in tivo desk top or anywhere else on the computer, except in the external hard drive which is listed in "my computer". However, after I transferred a few shows I got a critically low disk space message. The next day, it gave me a low space in C drive message. I don't know why the videos are taking up disk space after I've moved them, or how to prevent it.