chameleon , 5 hours ago

Easiest way would be to use borg create --read-special --chunker-params fixed,4194304 '/home/user/sdcardbackup::{now}' /dev/sdX (which I copied from the examples in the documentation). I'm not sure if Vorta has a way to activate --read-special but I suspect not; you can most likely still use it to make the repo and manage archives inside of it though.

Backing up from a command/stdin might also be relevant as an alternative, since that lets you back up more or less anything.