Sounds like you formatted the entire sd card (I assume you mean the internal card was what you formatted)...you need the Kobo image for your device to put back on the sd card. There is a specific thread where you can request this.
I think this is the correct thread for your device:
https://www.mobileread.com/forums/sh...obo+aura+image
Sorry @davidfor if I pointed to the wrong thread....