I recall seeing years ago a device which you would attach two USB drives to, and it would copy files from one USB device to the other. Usually two flash drives, but it would also work with sd to USB adaptive, and USB hard drives.
I can't find it again, and I want to be able to backup my photos by essentially duplicating my flash drives when they are full, and backing up to a larger USB drive.
Further, for a variety of reasons, I want to do this without using a computer. (speed, power consumption, target of theft, etc)
Any recommendations or tips?