Surely a copy (any copy, also a possible copy from the external hard disk to the internal hard disk) doens't happen spontanously and automatically. If that really happens, you must have that setup yourself. And that in turn means that you're the only one that can know to undo it.
Don't you turn off the PC and the external disk at night?
My computer's hard drive is very full. I have gone into my photos and videos and removed them to flash drives, deleted them and emptied the recycle bin. When I get up in the morning they are all back on there. I have an external hard drive and I suspect that overnight it is restoring them. How can I stop this from happening, but have them remain on the external hard drive as added security for their safety?