It’s going to be a short update this time. Little Backup Box now features a completely redesigned web UI. Whether it’s an improvement over the previous one is for you to decide. 
Currently, Raspberry Pi 4 has an annoying quirk that renders its fast USB 3 ports pretty much useless. If you try to use both USB 3 ports to back up a card to a storage device, Raspberry Pi unmounts the latter, which causes the backup to fail. This doesn’t happen if you use a combination of USB 2 and USB 3 ports.
I don’t know what causes the problem, but until it’s fixed, you can use a simple workaround. You can connect both the storage device and the card reader to the Raspberry Pi via an inexpensive USB 3 hub like Anker 4-Port USB 3.0 Hub.