The episode is "The Mind's Eye".

But I think the Star Trek writers just didn't understand computers very well. In the future, making duplicate copies of data seems to be impossible. When you copy a file from one device to another, or one ship to another, or transmit it to a planet, it seems to disappear from the source.

This is a particularly common weirdness in Voyager, where duplicating holographic programs is apparently impossible. You can only move them.