Gerry
I would question why you want to do this. I use a similar workflow, backing up at import. But this initial backup is really only a last-ditch final reserve, where I might go if I can't recover a specific image from either the main image store or either of the 'workflow' backups. In practice, I have used these backups only once or twice, and when I have all I managed to do was confirm that the original file was corrupt.
I just download everything and leave it. With hard drive costs now so low, I don't see putting in time to edit out some of the images as being cost effective. By doing this, I ensure that, aside from corruption of image files on the card or in the camera, there is absolutely nothing which could damage the file, so if this initial file is corrupted I can at least be confident that it wasn't anything I did (apart from camera or card abuse!). It also means that if there should ever be a reason why I should need to look at a neighbouring image which might not have made it through QC for technical or compositional reasons, it will still be available even though it hasn't been cluttering up my catalogue.
I use 2.5" HDs for this initial backup (we shoot in all sorts of weird locations, so portability is important, though I have a suspicion that the 2.5" drives are actually more robust than the 3.5" ones, which is an added advantage), and I just keep using the same one until it is full. I then set it aside and start another one.
Patrick Cunningham