They say that if you don't learn SOMETHING every day, then that day is wasted. You are absolutely correct about the creation date. I had to test it out to verify because I knew that the last modified date didn't change. Lo and behold, when you examine the properties of the file before and after copying it to a new drive, the creation date changes to the date and time you copied it. That's good to know, and especially good to know that B4A uses that date as criteria. Thank you for today's lesson!
If you want to continue with this solution, having the "created date" modified, you can set an exclude filter in Backup Properties->Filters, to exclude all files with "created date" older than the one you specify (the date when you move the files). This way, only files newer than that date will be considered to be backed up.
Or you can uncheck that comparison criteria from Backup Properties->Advanced page.
