Automatic backup on a second internal hidden disk when files are moved to a first disk

Summary: I need to automatically backup files on a hidden hard disk whenever the user copy files on a specific hard disk. Long explanation: Context: My father (70 years old) is very bad at computers. He barely knows how to copy and paste with a graphical interface, nothing more. I have installed him a linux mint distro which I have tailored so he doesn't crash it and understand easily. He use his computer mostly to save his photographies: around 2TB now... I plan to install him an extra hard disk called "photos" so he can discharge his photos on it. I also plan to put another disk ("photocopie") as a backup so in case the first hard disk crash, he will not lost his photographies. I will make this second hard disk hidden so my dad doesn't fuss with it. I would like to know how to automatically copy the new photos he adds on the hard disk "photos" to the second one "photocopies"; he only do how to do this using caja (Mate file manager). I had first read about using raid 1 but it doesn't seems to be the proper solution. Some suggested using rsync with cron, however cron work at specific times, and since my dad doesn't use his computer often, it may not work properly. I was trying to find an app that would start at a specific event such as "if files are written on hard disk photos, then run rsync as incremental on the hard disk photocopie", but I couldn't find an app detecting events. I have read about inotifywait, could it be the solution? How to implement that? PS: the photo sources could be anything than can be plugged by USB: a camera, an smartphone (Apple). My dad usually run caja and copy past from the phone to the proper directory/hard disk.