Try this: Attach an USB drive. Create a directory and subdirectory on the USB drive. Open two of Q-Dir window panes for this drive: one for the root folder and one for the subdirectory. Copy a few large files (700MB to 1GB each) to the root directory of the USB drive. Then copy one large file from the root directory to the subdirectory (drag and drop or copy/paste). Then try to copy another large file from the root directory to the subdirectory (drag and drop or copy/paste). I can't copy a second large file. If you duplicate this problem, perhaps you can debug.
Otherwise, a very, very good app you've created!
I will correct it! Thanks for the tip.