Losing source disk space while copying

I lose available disk space on the source drive when I copy a file from it.
Here is my setup. I have a 2nd internal data drive (500gb) and an external
750gb drive on a Dell 531 with Vista and 3gb RAM. As an example - I could
have 5gb available space on the source drive and 400gb available space on the
external destination drive. While copying a movie file, say 4gb, the
available space on the source drive begins to shrink. One of two outcomes are
1. The movie file successfully copies but the available disk space on the
source drive is at zero or much less space. To get it back I need to scan
disk or reboot. 2. The available disk space on the source drive runs out
while copying and I get an error message that says there is not enough
available space on the DESTINATION drive not the source drive.

Anyone have any ideas what is going on?

Thanks,
MJA