This will be kind of subjective to your application/system.
nerves_runtime has a uevent subsystem that gets published via system_registry.

You can subscribe to system_registry and get messages whenever there is a systemregistry change. Unfortunately working with system_registry kind of sucks sometimes. If your server dies, you have about zero chance of seeing what went wrong because of the log barf because of the size of the message. see #37 for more info.

on the other hand, on rpi3 when you plug a usb device in, it will likely show up as /dev/sdaX. you can just poll for that device to exist, mount it do your backup and un mount it. (i do this in production to some extent.)

thanks a lot @ConnorRigby! probably the second option is the best one for my case.