I assume that the remaining remnant files would be the resume-point, bookmarks and program description files that are visible via FTP, and on a USB drive. They are only tiny files so won't use much space (though the Humax should delete them). But if you want to be sure that they are deleted off the USB drive after deleting the recordings you could keep the recordings in folders on the USB drive. Then after deleting the recordings in a folder you can delete the folder itself to get rid of all of the little remnant files.
