I have a flash drive here I'd like to include in my time machine backup
Not a good idea.
You don't want to have TM back up a drive that will sometimes be unmounted. If TM runs while the drive is unmounted, it will record in its snapshot that the volume was not present. The next time TM runs after you mount the drive, it'll notice that the volume is new, and will back up the whole thing. Unmount it, let TM run, mount it again, and let TM run again, and TM will again decide that the entire volume is new and copy all files all over again. All this copying not only consumes time but also chews up precious space on your backup volume.
You're probably better off letting SuperDuper clone the flash drive to a sparse bundle disk image, and letting Time Machine back that up. A sparse bundle splits the disk image into 8MB chunks. When SD updates the image, chances are only some of those chunks will have changed, and TM will copy only those.
Be mindful that TM will not back up any disk image, sparse bundle or otherwise, while it's mounted. It treats a mounted disk image as "nothing has changed here". If you want TM to back it up, you need to occasionally (usually?) leave it unmounted.
